The coverage metric is the pff assigned coverage grade. The film of the game is reviewed and each player in coverage is assigned a grade for coverage on every play they are in coverage between -2 and 2. So, if the ball was uncatchable they don't care--they only care about how that player was covering their receiver or area. If you go by Passer Rating allowed the defender gets a bonus for uncatchable passes.
This article discusses the grading better than I can:
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www.profootballfocus.com]
One thing the grades provide that the data does not is that the human judge can rule in a binary--good or bad--way for each play independent of the surrounding action. In a way, that the data can not do. So, while one may disagree with how good or how bad there should not ever be a disagreement on good or bad. That should be obvious from watching the play.
