Are PFF grades weighted in someway? Here's an example of what I mean. Olineman A allowed X, Y, and Z pressures, QB hits, and sacks from Dlineman A. BUT Dlineman A has a mediocre grade so all those pressures, QB hits, and sacks weigh more than if they had came from Aaron Donald. We'd expect to give up pressures to an elite guy but not so much a random scrub.
If that is what is going on, then if your grades for the players are wrong, then your weights are wrong and thus your grades for your players are wrong.