You also mitigate your differential by not losing big to the other teams. In any case, it doesn't have to be perfect. It just has to split the teams and when players understand how fine the margins can be, the end of games will always have something to continue to compete for rather than letting it play out.
Outside of wanting to progress, it encourages another layer of intensity that wouldn't exist in a normal regular season game. Should make you fight harder to win your group too rather than leave it to fate.
I think he means ages 16-18.