There was a flash of light and Jon could feel his clothing changing. After the light faded and Jon could see again, he saw himself in the mirror wearing a football uniform. Biff must have been playing football at that moment.
Jon realized that he hadn't thought his whole wish through. It would be very weird walking around in football gear and not playing football. He decided since he couldn't reverse it he would just have to roll with it. Everyone else saw it as normal anyways. Right?
