say 'a' represents the first digit and 'b' the second...

each number will be like 10*a + b
for example 89 = 10*8 + 9

then you contract ( a + b ) -> 10*a + b - (a + b)

so with the example it becomes 10*8 + 9 - (8 + 9) = 72

if you work the 10*a + b - (a + b) out you'll get:
10*a + b - (a + b) = 10*a + b - a - b = 10*a - a = 9*a

so you'll always end up with 9 times the first digit
all the multiples of 9 have the same sign, which is also the one that appears in the crystal ball... :P