Where did you get the info on being top 15 in D 4 of the last 5 years??? I looked up some stuff and found this...
2016 - 12
2017 - 28
2018 - 18
2019 - 23
2020 - 51
Info was found here : https://www.sports-reference.com/cfb/years/2020-team-defense.html
There's other sites with comparative stats...