[edit] argh I opened up the posting window to make a post but got tied up with something else for a while, only to find that there was a new post there.

Anyway, StratComm is correct on that; any two digit integer can be written as 10x + y, where x and y are one-digit integers, so the procedure they instruct you to carry out is 10x + y - (x + y) = 9x, so you will always get a number that is a multiple of 9. The symbols on the chart are randomized on every try so that you don't see the same thing every time, but if you look carefully you will notice that all of the 9 multiples always have the same symbol, so it just displays whatever that is.