Which score? hub score or author score?
Anyway, if you're talking about each hub's score, it is:
1 - Because the hubs are that good
2. - Because the hubs have good keywords and well marketed so they receive lots of traffic, ratings, and comments
If you're talking about author score, it is:
1. Because the hubs have good scores
2. Because that person, though doesn't make many hubs, is still active in the forums.
3. Because that person rates and comments on other people's hubs
4. Because that person answers questions