One aspect of a good statistical defense is having a ball control offense on the field with few TOs. Another aspect is how deep you are, if your 2s are almost as good as your 1s, you'll at least have a fresh defense that won't be tired.
I was a bit surprised to see the Dawgs relatively low in that "ranking", but whatever. They have a lot of "D" returning, and their 2s play a lot.