You say ‘regress that far’.
Of their 10 wins last season, they won 6-7 close games that easily could have swung the other direction.
Perhaps they overachieved and weren’t really that good in the first place?
The regression you question might be as easy as a very tough opening schedule in 2020 and close games not going their way as frequently as they did in 2019.
They didn’t maintain or add to their team to the same degree as others via the draft and free agency.
Oh, and also just happened to lose one of the NFL’s top WR‘s that opponents typically game planned against. Not having that player drawing heavy attention on key downs, let alone moving the sticks himself, just might drastically change a team, you know?