1 This is the book of the generation of humans on the day when God made Adam, he made him according to the image of God. 2 He made them male and female. He blessed them, and he named their name Adam on the day when he made them. 3 Adam lived two hundred and thirty years and fathered a child according to his appearance and according to his image, and he named his name Seth. 4 The days of Adam after he fathered Seth were seven hundred years, and he fathered sons and daughters. 5 And all the days of Adam that he lived were nine hundred and thirty years, and he died. Genesis 5:1–5 (LES2) 

If we continue to compare these genealogies, we will discover that the MT removed a total of 1300 years from the LXX.

1 This is the book of the genealogy of Adam. In the day that God created man, He made him in the likeness of God. 2 He created them male and female, and blessed them and called them Mankind in the day they were created. 3 And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. 4 After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters. 5 So all the days that Adam lived were nine hundred and thirty years; and he died. Genesis 5:1–5 (NKJV) —

This passage provides an example of death by a thousand cuts. Assuming, for the sake of argument, that the LXX follows the original Paleo-Hebrew more closely than does the MT, as I showed in my article on why Christians should use the LXX, we see numerous departures in the latter. 

The LXX says that this is book of the genesis (Greek) or origin of men (Greek: anthropos). It says Adam himself was made in God’s image. That is a different wording and order from the MT, which. connects Adam with reference to a genealogy and mankind in general being created in God’s image. 

In last part of verse 2, the LXX uses an active verb – “he made them;” whereas, the MT uses a passive one – “they were created.” This sort of thing happens a lot.

Both the MT and LXX use the word “adam” as the name, but the MT translators chose to go with a general term “mankind,” probably because suddenly Adam is a plural “them.” This is a mystery that is clarified and fulfilled in the New Covenant. Eve was in Adam and was considered separate from him (“them”) while she was still “in” him. This foreshadows the mystery of the church, which was taken from the side of the Last Adam, our Lord Jesus. (1 Corinthians 15:47) We are blessed because we who believe are “in Christ,” just as Eve received a blessing while she was still “in” Adam. Therefore, the LXX is correct in identifying the “them” as singular Adam.

In verse 3, the MT shaves 100 years off of Adam’s age when he fathered Seth. Barry Setterfield gave reasons for this in one of his articles on the Septuagint. Below is a summary.

Therefore, the summary of the answer to your question is this: the basic reason why the Council of Jamnia changed the chrono-genealogies in Genesis 5 and 11 was to bolster Akiba’s traditions about Noah being the Second Adam and Shem being Melchizedek. In order to do this exactly, Akiba had to omit 600 years from Genesis 5 and 700 years from Genesis 11. He had the precedent of the Samaritan Pentateuch to back him up since the Samaritans had changed their text of Genesis 5 in a similar way in order to accommodate a strange tradition that they held, so he felt he could act in a similar way.

It is noteworthy that the lifespans of these early men were not shortened, only the time leading up to fathering their sons.

6 Seth lived two hundred and five years, and he fathered Enosh. 7 And after he fathered Enosh, Seth lived seven hundred and seven years, and he fathered sons and daughters. 8 And all the days of Seth were nine hundred and twelve years, and he died. 9 And Enosh lived one hundred and ninety years, and he fathered Kenan. 10 And after he fathered Kenan, Enosh lived seven hundred and fifteen years, and he fathered sons and daughters. 11 And all the days of Enosh were nine hundred and five years, and he died. 12 And Kenan lived one hundred and seventy years, and he fathered Mahalalel. 13 And after he fathered Mahalalel, Kenan lived seven hundred and forty years, and he fathered sons and daughters. 14 And all the days of Kenan were nine hundred and ten years, and he died. 15 And Mahalalel lived one hundred and sixty-five years, and he fathered Jared. 16 And after he fathered Jared, Mahalalel lived seven hundred and thirty years, and he fathered sons and daughters. 17 And all the days of Mahalalel were eight hundred and ninety-five years, and he died. 18 And Jared lived one hundred and sixty-two years, and he fathered Enoch. 19 And after he fathered Enoch, Jared lived eight hundred years, and he fathered sons and daughters. 20 And all the days of Jared were nine hundred and sixty-two years, and he died. 21 And Enoch lived one hundred and sixty-five years, and he fathered Methuselah. 22 Enoch was very pleasing to God for two hundred years after he fathered Methuselah, and he fathered sons and daughters. 23 And all the days of Enoch were three hundred and sixty-five years. 24 And Enoch was very pleasing to God and was not found because God transposed him. 25 And Methuselah lived one hundred and eighty-seven years, and he fathered Lamech. 26 And after he fathered Lamech, Methuselah lived seven hundred and eighty-two years, and he fathered sons and daughters. 27 And all the days of Methuselah, which he lived, were nine hundred and sixty-nine years, and he died. 28 And Lamech lived one hundred and eighty-eight years, and he fathered a son. 29 And he gave him the name Noah, saying, “This one will relieve us from our works and from the pains of our hands and from the earth which the Lord God cursed.” 30 And after he fathered Noah, Lamech lived five hundred and sixty-five years, and he fathered sons and daughters. 31 And all the days of Lamech were seven hundred and fifty-three years, and he died. Genesis 5:6–31 (LES2) 

1 And Noah lived five hundred years, and Noah fathered three sons: Shem, Ham, and Japheth. And it happened when humans began to become numerous upon the earth, and they had daughters, Genesis 6:1 (LES2) 

6 Seth lived one hundred and five years, and begot Enosh. 7 After he begot Enosh, Seth lived eight hundred and seven years, and had sons and daughters. 8 So all the days of Seth were nine hundred and twelve years; and he died. 9 Enosh lived ninety years, and begot Cainan. 10 After he begot Cainan, Enosh lived eight hundred and fifteen years, and had sons and daughters. 11 So all the days of Enosh were nine hundred and five years; and he died. 12 Cainan lived seventy years, and begot Mahalalel. 13 After he begot Mahalalel, Cainan lived eight hundred and forty years, and had sons and daughters. 14 So all the days of Cainan were nine hundred and ten years; and he died. 15 Mahalalel lived sixty-five years, and begot Jared. 16 After he begot Jared, Mahalalel lived eight hundred and thirty years, and had sons and daughters. 17 So all the days of Mahalalel were eight hundred and ninety-five years; and he died. 18 Jared lived one hundred and sixty-two years, and begot Enoch. 19 After he begot Enoch, Jared lived eight hundred years, and had sons and daughters. 20 So all the days of Jared were nine hundred and sixty-two years; and he died. 21 Enoch lived sixty-five years, and begot Methuselah. 22 After he begot Methuselah, Enoch walked with God three hundred years, and had sons and daughters. 23 So all the days of Enoch were three hundred and sixty-five years. 24 And Enoch walked with God; and he was not, for God took him. 25 Methuselah lived one hundred and eighty-seven years, and begot Lamech. 26 After he begot Lamech, Methuselah lived seven hundred and eighty-two years, and had sons and daughters. 27 So all the days of Methuselah were nine hundred and sixty-nine years; and he died. 28 Lamech lived one hundred and eighty-two years, and had a son. 29 And he called his name Noah, saying, “This one will comfort us concerning our work and the toil of our hands, because of the ground which the LORD has cursed.” 30 After he begot Noah, Lamech lived five hundred and ninety-five years, and had sons and daughters. 31 So all the days of Lamech were seven hundred and seventy-seven years; and he died. 32 And Noah was five hundred years old, and Noah begot Shem, Ham, and Japheth. Genesis 5:6–32 (NKJV) 

We see in this genealogy that the MT shaves 100 years off the ages of most of these men before fathering their cited sons. Jared and Methuselah are exceptions, for some reason, probably because the authors of the MT did not need their timeline to change to accomplish their purpose. In their cases the MT and LXX agree. 

Interestingly, the LXX uses the Greek word metatithemi to describe what God did with Enoch. It means to change places. He was removed from the earth and transferred to heaven.

Lamech is another exception to the rule. The LXX says he was 188 years old when he fathered Noah. The MT says he was 182. 

What the LXX says about Noah is extremely interesting. Noah is clearly a type of Christ in that what was spoken over him. The LXX says that he will:

• “This one will relieve us from our works,” bringing rest. Christ did just that, releasing us from having to work at pleasing God through keeping the Law. (Hebrews 4:8-10) The MT says Noah will “comfort us concerning our work,” a wording which does not properly foreshadow what Christ accomplished for us. 
• The LXX inserts an “and” in the sentence linking the relief we receive to the “pains of our hands.” The MT, as it typically does, translates this figuratively by saying Noah will comfort us regarding the toil of our hands. The Hebrew here can mean pain, as well. The Greek in the LXX only means pain. 
• Finally, the LXX says that Noah will relieve us from the earth which God cursed. The MT says the needed comfort that Noah will provide is because God cursed the ground. The New Testament says that creation itself will be released from its terrible bondage caused by sin at Christ’s Second Coming, when God will recreate it. (Romans 8:21)

According to Barry Setterfield, Akiva wished to establish Noah as a second Adam, a title the New Testament gives to Christ, the Last Adam. (1 Corinthians 15:45) Ironically, the LXX makes a better case for such a claim, but we know these things apply to Christ, to whom the entire Old Testament points, because Noah did not accomplish these things. (John 5:39)

The LXX says that Lamech lived a total of 753 years, but the MT says it was 777 years. 

The MT has Methuselah perishing the same year as the flood, perhaps in the flood, but the LXX has him dying five years earlier. Since Methuselah is part of the godly line of humans who eventually produced the Messiah, it is reasonable to assume he did not perish in the flood, God’s judgment on evil humanity.