>>pg+(OP)
It seems likely to me that this is just because "old timers" are seeing the same data as newbies. How many folks look beyond the first hundred posts, or even the front page for stuff to vote up? It makes sense that accounts older than a year will just slightly reshuffle the top 20 posts or so.
EDIT: It would be interesting to see this comparison in an alternate reality where we could downvote articles. I would bet that you would see more variation between newer and older accounts.