"I wish that for the remainder of the time my mom has the personality of Biff, my dad will have the personality of Sarah McMillan." The stone glowed, granting his wish.
He figured that since (in the previous reality) Biff and Sarah were a couple, it would make sense to have the same dynamic here as well.
But then Jon realized that he made a mistake with the wish. He meant to swap the personalities of his dad and Sarah, but instead he just wished that his dad had Sarah's personality, meaning that Sarah still stayed as herself.
He shrugged and thought Oh well.
