technically, you're wrong...it is possible, but i'm assuming that wasn't the case yesterday
obviously a crazy hypothetical scenario, but say a guy goes 5/9, and all 5 completions are screen passes that go 70 yards each for touchdowns, and he throws another 4 horrible balls that are dropped by the defenders...would you say he had a great game?
again, an absurd example of course, but the reasoning PFF gives for their grading system makes sense
from seeing some highlights, it was his timing and accuracy that was incredible last night, not necessarily the degree of difficulty of the throws and him dropping them into tight windows, which i believe is what PFF measures