Because this was The Random Stone, it wasn't granted to Zoe, but rather someone else within the 50-foot radius. And that person was Karyn, who was already in the hallway, trying to find who had the stone.
Moments earlier, the front door was opened by Mrs. Gibson (who looked just as athletic as Karyn remembered) and Karyn took that opportunity to dash into the house and up the stairs. She didn't have time to explain why she was there. She had to stop whoever was using The Random Stone. Thinking it could have been either Zoe or Michelle, she put her ear up to their bedroom doors to see whether they were talking about the stone, starting with Zoe's door. And that was when she heard Zoe making the Biff Meadows wish.
She rammed the door open and yelled "Stop!" But it was too late. The stone began to glow, causing all four girls to look away. The wish had been granted. To Karyn, specifically. But she didn't know it yet, since she heard Zoe's wish.
