I'm sure I misremembered; my mind is a sieve. But I've been doing some reading at HP Lexicon as per writcraft's suggestion, and I think what I should have said was, "If Slytherin beats Ravenclaw by so many points, then Gryffindor has to beat Hufflepuff by so many other points in order to win the cup!" Apparently - at least according to this essay (http://www.hp-lexicon.org/essays/essay-quidditch-almanac.html) - each Hogwarts team has the chance to play each Hogwarts team (resulting in six matches per year), and therefore the winner of the cup is based off of the points system in a "round-robin" fashion. :)