[cd34]

Chop will probably see the pattern right off. When doing bank reconciliations, it is very easy to determine that someone switched digits if the sum of the difference is 9.

This works on the same principle. Since any number minus the sum of its digits is a multiple of 9, all of the multiples of 9 contain the same symbol. They randomize the symbol so that it appears more mysterious, but, there's your answer.