I've wondered that.
As I couldn't work out quite what it did, I tend to tick to the 'View New Posts' option - though that's a bugger if you have missed a few days and have to leave before getting through all the pages, as it drops any 'non-new' next time, even if you haven't read them.
I'd love to know how it's decided - Mods - any offers?
More often than not it doesn't matter, but half the time I can't remember whether I've read something or not, and it's useful just to swing the cursor over and see if there's anything new.
I've tried several times to determine the sequence, things seem to get confused, or at least I do, by what is held in machine Cache and the UKW cookies.
If you clear your machine cache or Log Out you get different results than if you log in automatically each time.
So far I have not found anything in the forum documentation that explains what is expected to happen.