I participated in the contest myself, and I have to say the problems were very difficult for everyone at our contest site. Best team here got 2 out of the 8 questions solved. I'd be willing to bet the winning teams had graduate students, where a lot of teams can have sophmores or juniors and won't fare as well. Regions often have different questions as well because they compete at different times during the year. Kudos to the top teams, not an easy feat, many factors go into deciding who is the best.