Factors of nine. They change the symbols to hide this.
You have a two-digit number that is X*10 + Y
If you add the two digits and subtract them from the total, it looks like:
X*10+Y - (X + Y)
which is
X*10+Y - X - Y
which is
X*10-X
=
X*9
So it's the same as picking one number between 0 and 9, and multiplying by nine. Now the SWF routine puts a symbol in 0,9,18,27,36,45,54,63,72 and 81, and presents it when you're done.