The whole point of the mind reader is to perform the task off adding two digits then subtracting from the original to get a !!!MULTIPLE!!! of 9. Then have all multiples of 9 have the same symbol, so the person not noticing this, actually thinks the crystal ball is reading his/her mind.
Notice, the instructions do not say add two digits from a two digit number then subtract from the original until you get a number that when the digits are added equals nine. It would just be too obvious.
Ofcourse when you repeat the instructions far enough, you get nine
ex.
41
4+1 = 5
41 - 5 = 36
3 + 6 = 9
36 - 9 = 27
2+ 7 = 9
27 - 9 = 18
1 + 8 = 9
18 - 9 = 9
And this is the mathematical heart of the scheme.
But thats not WHY the crystal ball works.
The deception, trickery part of it, is from the multiples.
There is a difference between why and how.
MULTIPLES of nine. Thats the whole point of the game.
End of Story! Done. Discussion is over.