Additionally, I don't think you're taking the totality of the situation into consideration.
We're talking RGIII who has had every off-season that he was healthy, dedicated SOLELY to him. He was the focal point, receiving the majority of reps, attention and having the offense catered to his abilities.
On the other hand, we're talking about Cousins, who had the opposite experience. I'd like to imagine that players are aware of these things and judge/d them accordingly. We still haven't seen what Kirk can/will do with an offseason under his belt. An offseason where he is a starter and where we can pick/choose what players fit him. He hasn't had that yet and without it, he put up a strong season. So it's not a leap to think that he can further improve.
The run game average less than 3YPC for most of the 2015 season. Kirk was forced to throw 30+ times in a lot of games. So with consistency and backing of the coaching stuff, he grew a lot. And he showed that it wasn't flukish. We'll see where he goes from here. I'm not saying it'll be greatness but it'll be better than anything we've had in decades.