I took a little while…well, about 8 tries…to find out how this works. Pretty simple when you think of it!
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!