Like I’ve seen many NFL players say regarding those grades on the OL, “how can they accurately grade when they don’t know the calls?”
LaFleur said Patrick was the highest graded OL, for example. That grade matters. The PFF grades don’t matter. They don’t know the call.
I think it's easier than you think.
In all cases: Was a penalty called?
For the Center: Did you fumble the snap?
In pass protection:
Did you block the man in front of you?
If the defense ran a stunt, did you hand it off properly?
If it's a screen:
Did you get out there?
Were you able to get a hat a on a guy?
On a running play:
Did you block the man in front of you?
If you released to the second level, did someone come down and trap block?
Is the above perfect? Nope. But I'd guess only 5 (roughly) plays leave the grader guessing. With most offenses running over 50 plays again, those 5 aren't the worst.
As an alternative: think of PFF as opponent scouts. When the Packers watch film of the Bears, they don't know the playcalls either. But they can make reasonable deductions and grade players/plays.
Understanding what PFF tries to do and the limitations around it make it easier to accept when/how to use their grades.