Ok, the deal is every time you do that operation with any two digit number you get a multiple of nine, for instance, 67-(6+7)=54=4x9, or 38-(3+8)=27=3x9. It can be demonstrated with modular arithmetics in an easy way. So if you take a look, every multiple of nine has the same symbol assigned, and it changes to another symbol when you run the test again, which is what can "disturb" your mind at first.
Finally I am becoming stupider no more
-the epitaph Paul Erdos wrote for himself