I agree with mets. There is a bigger gap between C and C- to B range, compared to the differences between B/B+ and A-/A range. In other words, IQ matters a lot, so if you have a team thats not as good in ratings but are all A's and B+'s compared to a team with better ratings but only one or guys in the B/A range and the rest of his guys are C's, you have a good chance at pulling an upset.
Think of a veteran mid-major, with all Jr's and Sr's compared to the Kentucky's of the world with all Frosh. Kentucky may be more talented (probably is), but the team with the sr's has a great chance of winning the game. Okay, a very good chance.