I think it would be from the match ups. (Would really need a BB to confirm this) The team ratings aren't that important. The team rating is worked out from who is playing where. Rebounding match rating is worked out:
From the Game Manual:
Rebounding: Center and power forward weighted heavily, small forward somewhat heavily, both guards medium.
The truth is that what you see in the box score, or the match report, is only a fraction of what is going on inside the game engine.