USC is averaging 42.7 ppg, true. But that's against defenses giving up 29.8 ppg. Notre Dame gives up 20.3 ppg. You have to compare their average to whom their playing.
Will USC score 42 or more? Maybe, sure. But the odds are they won't. Anyway, they have real problems on defense. They are weakening late in the season, allowing all of their last 5 opponents to score more than their season averages. This bodes very well for ND to score more than their season average of 30.1.