Hi Dj Bueno
Nice one this had me baffled for a while, then it dawned on me. I will try and explain.
If you notice in the table that every multiple of 9 is the same symbol. ( eg 9,18,27,36,45,54,63,72,81,) are always the same.
Now any two digit number say (74) minus the total of the two digits(11) will always equal a multiple of 9 in this case (63)
Whilst it may seem like a mind reader, it is in fact a very clever trick.
I hope i have explained it clearly for you. Nice one!!