Guys... it's very simple! For every level in the hierarchy, it ranks the posts in order of points, and randomizes the order for those that have the same points.
it seems like, within "top level" comments with the same number of points, the comments switch between sorting ascending and descending by... a staggering of the time posted.
having more points puts you at the top of the comment list.
So, for example, this comment will have one point when i first post it. Since it's the latest comment, it will at first display topmost. When the next person posts a comment, my comment will display last and his will display first. Then when I refresh mine will display first and his last.
I'd assumed it was an artifact of the storage mechanism that pg didn't consider problematic enough to fix. PostgreSQL, for example, has no order guarantees unless you explicitly call for them, but seems to order by last update to the row.
Not that I'm suggesting that a SQL database backs news.yc; pg seems to have a preference for filesystems over databases.