The number has 2 digits - let's call them x and y. The positive difference between the two combinations (lowest-highest and highest-lowest) is always equal to the positive difference of x and y, multiplied by 9.
For example:
76 and 67: 76 - 67 = 9 and ((7-6)*9) = 1*9 = 9 73 and 37: 73 - 37 = 36 and ((7-3)*9) = 4*9 = 36
