I think it's more nuanced.
PFF analysts can be reading certain plays wrong and bad teams tend to have lower ranked players because the guys next to them suck.
Example: The center was supposed to block the nose, but PFF gets it wrong an assigns the blown block to the guard. If that happened over and over, it lowers the guards grade.
The next season the team upgrades the center and lo and behold the guards grade jumps by ten points.
I've looked at their grades for some of the Cardinals good players and I thought their grade was too low from what I've seen. This would account for why that happens.
I don’t think he’s that much of a nuance guy.
I literally said in one of my first posts on this that PFF measures performance, not ability, and that performance is influenced by a number of factors. For example, coaching and surrounding talent and that when using it as a tool you have to factor in context.
It's why I wanted Casey Hayward despite his poor grade last year. I don't think he suddenly forgot how to play football, or that he has suddenly physically declined from a CB capable of an 80+ grade in 2019 to one with a sub 60 grade in 2020.