The symbol I ended up with gave the following numbers next to that same symbol 98, 93, 81, 80, 72, 69, 63, 61, 54, 45, 36, 34, 33, 30, 27, 18, or 9
If you look at the numbers above, you should notice that all of the numbers in you nine times table (9, 18, 27, 36, 45, 54, 63,...) are all appearing under the same symbol. THis is because a multiple of 9 is the ONLY possible answer, no matter what number you chose.
For example, lets say you chose 80.
8 + 0 =8
80 - 8 = 72
But then if you look at the answer for choosing 81
8 + 1 = 9
81 - 9 = 72
It always gives a multiple of 9
This program has no way of reading your mind (hint for next time: computers cant read your mind), all it does is allocate one symbol for every value of 9 (as well as a few other numbers to make it look inconspicuous) and this symbol will also appear in the crystal ball.
EDIT: I type all that now you have worked it out... typical