Ooooh that thing. Yea it works because there are only a certain amount of numbers you can actually get when you do it. The pictures change every time and all the numbers that you could possibly get when you take a 2 digit number add them then subract it from the original number all have the same symbol.

For example if you pick 23 as your number. You end up with 18. You find 18 on the symbol chart and its lets say a cross. You click the crystal ball and it shows the cross. The next time look at 18 again. It will have a different symbol but it will be the symbol that the ball shows. All the numbers you can get will have the same picture so its always right