Tiffany ran after Sarah, but wasn't able to get to her in time. Sarah ran out of the house, as fast as it was possible, wearing high heels like that.
Usually Tiffany would continue chasing after her, but she didn't want to risk someone seeing her. Yes, she was wearing a skirt, but they would still see the bulge.
Of course, unknown to Tiffany, only she, Biff, and Sarah knew that anything was different, since only the three of them heard the wish. The same applied to Biff's vagina.
She walked back into Biff's room.
