Taking PFF as fact is where a lot of people get screwed up. These grades are based on the opinions of one or two guys who watch a replay of the game. You and I could watch a replay and come to different conclusions. Here is a piece of
an article on PFF that has some quotes by Bill Belichick.
Some plays are obvious. A WR drops a pass. A QB overthrows a receiver. A LB misses a tackle. But there's more nuance in a football game than can be determined by people that aren't privileged to the assignments of a particular player. If Bill Belichick can't say for certain what happened on a play, how can some guys watching a recording do it? Take the grades of PFF with a huge grain of salt. What we can determine is whether a team is successful or not. And the credit or the blame can be pointed at those who built and coached the team.