http://www.bored.com/mysticalball/index.htm

I suppose there's an obvious solution as to how it's done. This isn't the first one of these I've seen, but I can't remember how the other one went.

Could someone enlighten dumb-ass me?

The two numbers, added together and then taken from the original will give you a multiple of 9. All the multiple of 9's (9,18, 27, 36, etc.) all have the same symbol.

It resets each time, so they are kind of random looking, but they aren't.