Can you explain to me why there is such a difference in the Genesis 5 and 11 chronologies of the antediluvian and post-flood patriarchs? I understand there is about 1380 years difference between the Septuagint and Masoretic translation texts, and the Samaritan translation is different also. There can only be one right version. The other two are corrupted for some reason other than mistranslating them. I would think the ages of those listed when their sons were born would be fairly easy to translate.
I am not sure exactly what you mean by the “difference” between the pre-flood genealogy of Genesis 5 and the post-flood genealogy of Genesis. The two genealogies cover completely different sets of male descendants, so I do not see how there can be a discrepancy between Genesis 5 and Genesis 11. Perhaps you misspoke here and perhaps you can clear up what you mean when you say there is a 1380 year difference between the two accounts of different genealogies.
Let me guess. Maybe you mean that if we compare Genesis 5 in the Masoretic and Genesis 5 in the Septuagint there is a difference. Is that what you mean? Or perhaps are you talking about a difference in Genesis 11 between the Septuagint and the Masoretic or Samaritan texts. And, if we are talking about two different genealogies, where do you get the figure of 1380 years? Can you please clarify your question and where you get your numbers? [Editor’s Note: He does so below!]
Because your question is not clear, let me at least try to give you a generic answer. I am sure you are aware that there have been copying errors over time between the autographs and the oldest manuscript of the Septuagint and/or the Masoretic texts. Our Hebrew text has mistakes in it due to copying errors. Generally, the amount of change is remarkably small, but there are copying errors in our current best texts. The case with numbers is particularly problematic. The Jews used a letter-based number system, similar to Roman numerals. One major problem is that some of the Hebrew letters used to express numbers are really quite similar in appearance, making copying errors very easy to happen.
Add to this the fact that words are far less likely to have a copying error than numbers. For example: Which is correct: bought or bouxht? That is an easy question, because bouxht is not a word. On the other hand, which is correct 5780 or 5790? There is no way to know. Unlike words, which copiers can self-correct rather easily, copying errors in numbers are far more difficult to detect and to correct because of the fact that every number is possible, but not every letter combination is possible.
For this reason, the numbers we have in our Hebrew Bibles are relatively unreliable. Did Enoch live 650 or 850 years? (I am making up the numbers, just to make a point). The honest truth is that numbers of soldiers or numbers of years a king ruled or numbers of years a person lived is fairly unreliable in the Hebrew Bible. It is no surprise at all to biblical text scholars to find significant differences in the ages of persons in Hebrew chronologies such as in Genesis.
Two points about this: 1. Although the numbers in Old Testament texts are fairly unreliable, as a rule, the age of a person or the number of soldiers in an army have little to no importance as to the meaning of the biblical text. Does it really matter to the Christian how long Lamech lived, or how many soldiers were killed in a particular battle? The answer is a definite no.
2. Although the precise numbers are really not all that important in the biblical text, one might be able to speculate that the Masoretic text is more likely to be accurate than the Septuagint text, as the Septuagint is a translation. But even that is debatable, as the Septuagint translation was made about 220 BC, whereas the Masoretic text comes from about AD 800.
So, the difficulty of accurately copying numbers in a biblical text is probably the correct answer to your question (although I cannot comment on your specific question unless you clear it up as I request above). Please take the numbers in the Old Testament with a little grain of salt and do not make strong arguments about things related to the length of reigns of kings, the ages of people in genealogies and the like.
I hope this makes sense, and please consider replying to clear up your original question.