[gtc]

First, the grid is set up so that any number you pick will give you the same gift. For insance, if box 9 has Keychain, 10-0-0=9 and 11-1-1=9 and 12-1-2=9 etc, so they all give you the keychain.

Then it rearranges the grid each time, so you get a new gift if you try again. You think that because you picked a different number you got a different gift, but it's really just because the gifts are in different boxes. Most people will be too amazed to notice that the gifts are in different boxes.