My favorite 'where did that come from' defense was Arizona's Desert Swarm in 92-93.
Usually, the best run defense in the country gives up just under 100 yds per game, maybe as low as 90 or 80 - all-timers a little less.
In 1992, Arizona allowed 65.1 yards rushing per game @ 1.9 per carry. That's nuts, off-the-radar, crazy good. And the team went 6-5-1. That's awful, considering.
So we go to 1993, see if they can maintain that level of defensive dominance. No, but they improved on it! 30.1 rushing yards per game allowed!!! It's impossible! And at 0.9 per carry, they allowed fewer rushing yards than opponents had carries!!! It breaks my brain.
That squad went 10-2 (how did they lose 2???) and shut out Miami in the Fiesta Bowl.