PFF is pure garbage in, garbage out. They are getting freelancers on their couches, watching broadcast angles, to make snap judgements on a play while the game goes on. How can a casual fan who sees a play for 3 seconds tell if a DE got controlled by the blocker at the line or if his job was to set the edge? He doesn't. Dude's got no idea. Then it's a mark on a sheet, 30 seconds later, and it's off to the next one. Mid week, they have an All-22 review, where the amateurs repeat the same process with a different view and no more insight than before. They average those two numbers up and bam, you've got some grades.
They end up with a very official-looking table of numbers, and there's a tendency for people to take anything with enough numbers or decimals as scientific certainty without really understanding the process behind it (the term in the science world is "garbage in, gospel out"). But look at how they get to the "74.1, #17 edge rusher" type of claims, and it's really just an amalgamation of amateurs spitballing hundreds of guesses