As I said, it's a very old ball, with a multi-piece construction. Basically, where the 12 is, there are very faint edges. And from what I recall from my physics degree, edges tend to diffract light, which would result in occasional jumps.
It's really only this one ball that does it. I just think it looks the best of all the pool balls I have. I tend to swap back and forth between the default ball and this 12.