That may be because the Colts' are one of the only offenses in the league that NEEDS a GREAT QB.
If you look at the recievers, they aren't great YAC guys. Now, that's not taking anything away from Harrison, Gonzalez, and Wayne, because they are great at what they do(getting open), but without Manning, they would be trash. So, because of this, Manning needs to be more accurate further down the field(so the Colts' offense can get first downs) rather than doing a dink and dunk passing attack.
It's the reason Kurt Warner had a resurgence(though, he has awesome pure QB skills regarding accuracy, anyway)[Larry Fitz]. It's the reason Favre had a spectacular year a couple of years ago, or should I say his recievers...
QB play isn't just the QB's individual stats. How much of a QB's stats is mostly the QB and how much of it is reciever play is where you differentiate the great QB's from the okay ones.