This is the problem with PFF, they are selling their own subjective opinion as objective fact just because they came up with a grading scale so they can assign everybody a number.
They look at each play and give + or - based on the outcome. The problem is it's not that simple. You watch Ike get beat in one on one coverage and they give him a -. You watch Richard Sherman break up a pass and give him a +. Seems reasonable except there are too many variables left out.
Last year Ike was in many no win situations. They'd ask him to play the other team's top WR often times with no safety help or poor safety help. That changes how tight you can play a guy.
It's easy to see Sherman playing so tight on a WR and think he's the greatest ever but you have to realize the reason he can play that tight is because he has outstanding safety support and a great pass rush. Sherman doesn;t have to worry about double moves because the QB won;t have enough time. He doesn't have to worry about giving up deep balls because his Safeties are so good behind him. This means Sherman can play as tight as he wants and take big chances. It's a huge advantage.
Contrast that with Ike and he had a weak pass rush and no safety help. He has to worry about double moves and getting beat deep because he has no help much of the time. That means you can complete underneath throws against him at a high % and there's not much he can do about it. He can't jump those routes.
There is a reason why baseball is a stat sport and football is not. In baseball you are looking at 500+ at bats so it's a fair comparison to rank players by stats because in a full year, they've all faced roughly the same pitchers in each league.
Football doesn't work that way. You have to account for opponent, scheme, injury, and the players around them.
