Do you think that represents a difference between Henne and Tannehill's performances? If so, why isn't it reflected in their statistics?
Because football is the ultimate team sports and individual stats are greatly skewed and therefore almost completely worthless when evaluating players at individual positions.
If you want to apply advanced metrics, stick to baseball. The stats are still effected by some team variables, but not nearly as much as in football. For the most part it is batter vs. hitter with some outside variables coming into play.
In football, playing QB, you can put up some decent comp % numbers by checking down almost every play, taking sacks, and basically not having a clue how to advance your team down the field, put it in the endzone, and WIN.