That's because USC has had to play against far better offenses than Texas has. They've played against 4 of the nation's top 10 scoring offenses -- Oregon, Arizona State, Notre Dame and Fresno State -- and are facing a fifth next week in UCLA. That's not going to help your defensive stats like playing teams like Rice, Baylor, Kansas and Oklahoma State would.
Of course, Texas has the top scoring offense in the nation, so that would suggest another close game...but USC is also going to have four weeks to prepare for them, as opposed to October where they played three huge offenses in four weeks, with no time off.