http://milaadesign.com/wizardy.html

I got it

If you pick 2 digits X and Y then your 2 digit number is 10X+Y

so add the two digits = X+Y

and subtract fron the 10X + Y = 9X

so the number in the matrix you're looking at is 9 or a multiple of 9

If you look at the matrix, the symbols corresponding to 9, 18, 27 , 36 etc are all the same.

Sorry to spoil it

All multiples of 9 add up to 9 like 18 1+8, 27 2+7 so it must work every time.