OK, i'll tell you all how it is done.
I'd not seen this before, but i noticed there was a connection.
Its simple if you realise.
Take any 2 digit number, say 35.
Add the 3 to the 5 and take that from 35 which = 35 - 8 = 27.
This would give you a symbol.
OK, now say we choose a different number say 78.
78 - 15 = 63.
Do we see any connection yet?
No, ok, other numbers.
45 - 9 = 36
68 - 14 = 54
It becoming Clear yet?
so, the numbers so far are 27, 36, 54, 63......these numbers have one thing in common, yes, they are ALL multiples of 9.
Don't think thats it. OK, lets try all the numbers in the 30's
30 - 3 = 27
31 - 4 = 27
32 - 5 = 27
33 - 6 = 27
34 - 7 = 27
37 - 10 = 27
38 - 11 = 27
39 - 11 = 27
All the designer has to do is make sure to have the same Symbol on every multiple of 9 and it is guarenteed to pick the correct one you arrive at.
All the other symbols are so random that they make it look random.