...and this I STILL don't understand if it's a SELECT...LIMIT type command. It just... shouldn't, it's against every sane programming practice in the world. I dunno.
That's what I thought - the data it actually fetches is just the posts it displays, and it'll just SELECT COUNT(*) to see how many posts there are in total, in order to generate the right amount of pages.
Just a guess anyway, I've not seen the code. Makes sense though, although I've heard lots of stories about the vB devs being bemusingly incompetent.