81, 63, 45, 27, 9 will always show the picture it's about multiples of 3 i could write a mathematical formula for how it works but i'm not going to it's late and i'm not a math person just a patern recognizer something about the way i think i'm also good at voice patterns writting paterns blah blah blah.
yet i cant spell eh
but since i'm bored and i can explain the zero easily it's because if somebody was like i'm going to be cleaver and try to fool the magic ball by not picking a 2 digit number (so if you take a one digit number and minus it from itself you get zero)
and one more time since i'm bored if you wanted to write this up for yourself and make it more fancy you could have all the symbols change as long as the multiples of 3 are the same symbol and then all the rest are just randoms. this site has fixed symbols for every number that isn't a multiple of 3(note again i'm not a math person so if i'm not using the right term then eh what ever)