lol its not that clever. Basically there aren't that many possibilites of answers you can get. There is one possibility for each set of 10 numbers. For example, between 10-19 the magic number is 9 as shown below:
10: 1 + 0 = 1, 10 - 1 = 9
11: 1 + 1 = 2, 11 - 2 = 9
12: 1 + 2 = 3, 12 - 3 = 9
etc etc. The same thing happens for numbers between 20-29
23: 2 + 3 = 5, 23 - 5 = 18
24: 2 + 4 = 6, 24 - 6 = 18
25: 2 + 5 = 7, 25 - 7 = 18
and so on. The number for 30-39 is 27 then 40-49 is 36 and so on. Spot the pattern?
Now basically all the possible numbers it could be have the same pattern. The other numbers just have random patterns. To make it slightly more convincing though, each time it uses a diff pattern to be used for the magic numbers, and randomises all the others differently. Therefore when you see it has picked a different symbol the next time you do it, you are amazed.
If you look at the numbers 9, 18, 27, 36, 45, 54, 63 etc etc then you will see they all have the same symbol representing them every time.
Not so psychic afterall eh? 🙂
[EDIT: I spotted that after about 20 seconds....nobody fools me muhaha! 🙂]