Hrmm... should work all the time...

Don't read this if you want the "mystery" to be alive

./start nerd

Pick a 2 digit number:
Call the number N = 10x + y (i.e. 28 = 10 x 2 + 8)

Add the two digits together (call this S):
S = x + y

Then subtract the addition from the original number (call this Z)
Z = N - S = (10x + y) - (x + y)
=> Z = 9x

./end nerd

So the answer should always be 9 times the first digit in the number you have selected. And if you look in the chart all the 9 times table values are the same.

Ok my head hurts now... Its too late for maths