Not a problem chaps. Don't read this if you don't want to know how it works!
Things start becoming a little clearer if you pick the same number each time. You'll find that the symbol you get changes, and you'll think "hmm, that's wierd".
Now, if you look further, you'll find that they symbols are the same for any multiple of nine, i.e. 9, 18, 27, 36 etc. This is because whenever you add the digits of a two digit number, and subtract the result from the original number you get, that's right, a multiple of nine. It's actually impossible to get, for example, 12, as your answer.
Hope that's reasonably clear - I was never much good at mathematical proofs!
Sure there's a more technical way of explaining it!