Ushijima and Bokuto pretty much score half the points in one set by their spikes in canon, so yes there are difference in skills in volleyball too. But they can't do it without their team, just as each GoM member can't play a whole game by themselves. I don't know if you read KuroBas, but in the end team play trumps individual skills, it's kind of the point of the series. :)

Of course being a sport school doesn't guarantee you to win. And I'm not saying that it's unlikely for a law/science/art school to win either. I'm saying it's unlikely that they keep winning. If you see in the list, the winners all alternate between schools.

I stand voting on random on everything.