The "assumptions" are based on how teams actually did on the court. Conference teams lose a bunch of games during the non-conference and it hurts your conference's strength. Win a bunch of games, and it helps your conference. The committee didn't "assume the ACC was bad" and "assume the Mountain West was strong", they placed teams where they belonged based on their resumes (and as always, "Teams" get bids, not "conferences").
Anyways, I've hijacked Bac's thread enough with this topic so I'm checking out on it.