Great fun while it lasts, though! Try it on children, especially, and hear the incredulous "HOW....?"
For any two-digit no., the number minus the sum of its digits is always a multiple of 9. So they just assign a symbol to all the multiples of 9 below 90 (it doesn't work after that)...and that's what you will see. Just to prevent you from spotting it, they assign a different symbol to the multiples each time you try it!
So next time you try it, look at the symbol assigned to all the multiples of 9 on the chart!
Neatly done, and very well packaged!