Actually Texas am is last in the sec in total defense.
That's total defensive yardage. A&M's played one more game than almost everyone else in the conference (they're just now getting to their second bye week; they, LSU, and UK have all already played 11 games to everyone else's 10).
If you look at average yards given up per game, they're only about 2 yards/game worse...with Carolina having played one less game.
Not to mention South Carolina has given up more average points (USCe is giving around 33 points a game - it was around 35 before the UF game, I think 36 without their FCS game - while A&M is only giving up about 27-28 across 11 games).
USCe has also given up more TDs (41 vs 39) while having played that one less game than A&M (granted USCe has also drastically scored more, with almost the same number of TDs in 10 games as A&M has in 11).
Also A&M's defense sacks the QB on average 2.91 times a game (32 total in 11 games) and averages 5.7 tackles for a loss per game (63 total).
South Carolina, again, has 9 total sacks in 10 games (less than a sack/game), and a grand total of 36 tackles for a loss in them (3.6 per game...they're next to last in the country that stat).
(They're honestly about the same in turnovers forced.)
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Long story short, their total yards are higher than USCe's because A&M's played one more game than everyone else, save LSU and UK.
Every other statistical category that involves an average over the number of games played, USCe's defense is much worse (save one - average yards allowed per game - where they're actually better... by 2 yards).