Well in those numbers they do not count dropped passes. and as anyone who has watched every snap of his last 7 starts will tell you that adds up to at least 2-3 per game. So when you are only throwing 10-15 times a game that really effects your %.
Dude. It's all relative. The Broncos aren't the only NFL team with receivers who drop balls. And a percentage is a percentage. One could argue that with Tebow throwing LESS, it will only HELP his %