Possible explanations include either 1) that the numbers were corrupted, possibly when being converted from a sexagesimal system into decimal, or 2) that the author never intended the numbers to be "real" numbers in the sense we would understand them, but was instead intending to use a pattern to teach symbolic meaning. However, to my knowledge, nobody has ever offered a completely convincing interpretation of the puzzle. If #1 is correct, what were the original values and what was the process by which they were corrupted? If #2, what is the pattern that the author was constructing, and what is it intended to communicate?
Other genealogies in Genesis have similar weird features, and some have proposed theories that involve the other genealogies as well, but that's beyond the scope of this comment.