I've got some that slip in and out of alltheweb for no logical reason. One page went from #21 to disappearing, up to #1 for one phrase & #4 for another, back down again and now it's there for one search and totally gone for another, with other pages from the site disappearing altogether.
On that site, first there are a lot of pages in, then only one obscure one can be found. It looks like something shifting periodically with no rhyme or reason. I sure can't figure it out, I just keep using the free submit when they're gone.