I wont pretend to know how PFF comes up with their grades but...
Doing Just What’s Needed
If you just looked at Peyton Manning’s (+6.2) numbers, you would think the quarterback is better than ever. A few plays in this game made it clear that Manning still isn’t 100% including his second pass in the game that missed Jacob Tamme and was nearly intercepted. Earlier in the season we would see many more of these kinds of mistakes, and that led to the Broncos digging themselves a hole they had to try getting out of. What Manning did do was avoid those blunders for the majority of the game, and when he did have a poor throw, it didn’t cost the team. There were a few highlight-worthy plays when Demaryius Thomas (+6.6) made the right moves prior to the catch to get open and Manning found him. For the most part Manning was able to complete short passes where he relied more on his decision making than his arm to convert first downs.
That has to be the most negative outlook on what he did Sunday possible. From that write up, you'd think the Broncos won in spite of him.