Four thoughts:
1 - the Zone Coverage rating governs instinctiveness, such a player’s ability to break on the QB’s throwing motion, in addition to their ability to relate to receivers spatially while in a zone coverage assignment. Jamal Adams is widely regarded as an instinctive and extremely intelligent player. Ask anyone who studies football about this, they will consistently tell you Adams has a great feel for the game. His weakness has always specifically been man coverage; he is not great at keeping up with slot receivers laterally because he doesn’t have elite short area quickness.
2 - PFF’s coverage grade makes no attempt to distinguish between man coverage and zone coverage. It is unreasonable to use PFF’s solitary coverage grade as a proxy for both skills.
3 - last I was aware, PFF’s completion blaming makes no allowances for the offensive route combination or defensive coverage calls. For example, if the offense calls Sail and the defense calls Cover 3 Sky, PFF’s grading makes no allowance for Adams being conflicted by a high-low stretch of the Sail concept, it only cares that he was in proximity to the receiver when the receiver caught the ball. This lack of context is probably a very bad way to grade how well a defender is holding up in coverage; it is unreasonable to expect a defender to win in a no-win situation.
4 - PFF generally has been talking out of both sides of its mouth with respect to its coverage grades recently. They published
a written article this offseason naming Trevon Diggs as the best press-man cornerback in the NFL, but also they gave him a 2021 season coverage grade of 58. Those two evaluations wildly conflict with one another.
Generally, we really need to start approaching these conversations with a little more nuance than “this one subjective number says the player is bad, so he must be bad”.