I've seen Riley try to throw a ball into a garbage can 30 yards away standing next to the other QBs. He uses his whole entire body and a couple steps just to muster up the strenght to get it there. Max did not have to do that, Jake did not have to do that, I haven't watched footage of Ty throwing the ball lately, so I can't speak to that.
That is Riley's problem, you can't be accurate when you are relying on every muscle in your body to get the ball far enough, odds are, one of the muscles is not going to react correctly every time, and that makes every throw different.
It's like trying to hit the ball too hard in golf. Sure you can get the clubhead moving really fast, but if you don't have a consistent, controlled swing, you are going to be landing all over the place. Or a 10 year old that barely has the strength to get a 3 pointer to the rim, but can't do anything consistent enough to ever make 2 in a row. Riley doesn't have the arm to have a consistent controlled throw. However, Max and Jake had that.