I think you may be making the mistake of viewing these puzzles as purely mathematical instead of also psychological. Mathematically its true, of course, that an

I think you may be making the mistake of viewing these puzzles as purely mathematical instead of also psychological. Mathematically its true, of course, that any number can come next in an arbitrary sequence. But its not true that each of the possible interrelations between those numbers is equally likely to be selected by human puzzle designer (*especially a good puzzle designer who is attempting to make the puzzle fun, challenging, but not impossible). Of course, that doesn't mean there is no such thing as a bad puzzle...