I still do not understand how the ground cannot cause a fumble, but it can cause an incomplete pass.
If still in possession of the football, you're down as soon as a knee or forearm hits the ground. Whatever happens after that doesn't matter, including the ball coming loose. When catching a pass or, in this case, a fly ball, the catch must be completed. Either you demonstrate control of the ball long enough to do something unrelated to the catch, or if going to the ground while making the catch, maintain control through contact with the ground.