The Hebrews took from the Egyptians a great love of numbers and numerology. 969 probably rolled off the tongue really well in their language. It was all originally passed orally in verse.

I once took up the Ussher chronology thing, wading through the begats, just to see if there was any sort of basis on which a person could legitimately convince themselves they were doing a proper chronology. Turns out that 969 means ol' Meth drowned in the Flood his grandson was known for - or at least kicked off in the few months preceding it. I'm thinking "drowned in the Flood" is something you're supposed to carry away from the tale, in much the same way as he's listed as somebody's son and somebody's father, without any accomplishments worth mentioning, nor any notable piety. There's probably another part of the tale, now lost, that folks were originally supposed to know.