It's no different than PFF trying to "grade" an O-lineman. It's literally an impossible job without inside knowledge.
They can make assumptions and get close, but it's not always going to be accurate.
Kelly the Dog makes some other good points somewhere too about film watching... Some pass plays have designed targets regardless of the availability of other receivers on the play, for various reasons. Or when guys "come open" on film after the QB is already winding up to pass somewhere else...