Even if we discount the fact that quarterback rating is an overrated stat, a good quarterback rating in college does not necessarily transfer into success at the pro level. If someone is playing on a great college team, the average opponent defense will be far inferior to your offense and your skill players will be able to dominate an opponent more often than not. Playing on a team that runs a pass heavy offense, especially if they run a lot of short, quick routes will up the rating too. When you remove the system benefits, a supposedly great quarterback can become very average.
I know this is an apple to oranges comparison in terms of style, but here's an example:
J Russell (college stats) See QB rating:
http://espndb.go.com/ncf/player/profile?playerId=146755
J Russell (pro stats) See QB rating:
http://sports.espn.go.com/nfl/players/stats?playerId=10446
I'm not saying that Tebow isn't going to be a success in the NFL; he certainly has the work ethic. However, he has greatly benefited from being on a great team that uses short, quick routes (the high completion percentage boosts the QB rating).