wow this thing is crazy

It's simple really. Take any 2 digit number, let's say 48. Do the equation, and you get 48 - (4 + 8) = 36. Let's try it again, with a different number, say 79. 79 - (7 + 9) = 63. What do 36 and 63 have in common? they're both multiples of 9. In fact, any time you do the equation, you'll automatically get a multiple of 9. Now look at the chart and look at all the multiples of 9, like 9, 18, 27, 36, 45, etc. Notice something?

it only worked once for me

If it didn't work your not adding and subtracting correctly because you should always come up with a multiple of 9 like ermitonto said and all the multiples of nine (the only possible answers) all are assigned the same symbol.