This is how it works.
 

I was always the kid who had to figure out how the toys worked and then when I dissected it, it lost its magic. So anyway, I did it again here. Theres a diagonal pattern by 10's and that's cross referenced with the shapes corresponding with the numbers that are subtracted to give you your....you know what...if you want to see my graphs I made to figure it out, ask me. Otherwise I won't go into detail on ya. The graphs break it down better than I can explain it.

It's real quite simple any number you pick ends up as 0 or a multipiable of 9 and they all have the same symbol.

When you add the two digits together of any group of 10's you come up with the same number.

If you pick any number in the 80's will come up as 72 because even if you go up or down you adding or subtracting 1 to each side.

example
89 8+9=17 89-17=72
88 8+8=16 88-16=72
87 8+7=15 87-15=72
86 8+6=14 86-14=72
85 8+5=13 85-13=72
84 8+4=12 84-12=72
83 8+3=11 83-11=72
82 8+2=10 82-10=72
81 8+1=9 81-9=72
80 8+0=8 80-8=72