[Sticks]

A friend of mine e-mailed this explanation

It will always show the (selected sysmbol)

Any 2-digit number can be characterized as 10t+u, where t is the 10's digit,
resp. u the units.

The instructions are to compute 10t+u - (t+u)
Which is 9t + t + u - (t+u)
Which is 9t +t+u-t-u
Which is 9t

So your final number will certainly be one of 0, 9, 18, 27, ... 81

Peruse the chart: you will see that all of these have the (selected sysmbol) associated.