Every subtraction you make brings you back to a multiple of nine... Simple example
10.....1+0=1, 10-1=9
11.....1+1=2, 11-2=9
12.....1+2=3, 12-3=9
13.....1+3=4, 13-4=9
14.....1+4=5, 14-5=9
It's the same way the whole way through the chart, and all the multiples of nine have the same symbol, from 0-99... All the other symbols are there to mess you up, that's the same reason they change positions every time it reloads. Point being, anyway you subtract it, you'll end up with a nine or a multiple of nine, and all the multiples of nine have the same symbol..Therefore, it always knows it's right... As long as you know how to subtract
*Edited b/c I can't spell to save my life