Ok here goes hope this makes sence:
Every answer you come up with is a multiple of 9
ie:
number 18
1+8+ 9
18-9 =9
number 45
4+5=9
45-9 = 36
Number 73
7 + 3 = 10
73 - 10 = 63
If you look at all the numers that are a multiples of 9 (9, 18, 27,36,45,54,63,72,81) they all have the same symbol
The only reason the symbol chart changes each time you do it so you never get the same symbol twice.
And there is your answer