If you do the math for, say 99, you'll come up with 81. If you do the math for another number during the same turn, it'll come up with the same symbol. It's all in the math ;). The symbols on the list do change every turn, though.
You always end up with multiples of 9, and all of the symbols to those numbers are the same.
I've seen a better version once on TV that included words, but it only works in german.