Odd eyes seems to be most common in baldheaded birds. Many of my baldhead rollers had one bull (black) eye and one white (pearl) eye. Piebald also seems to be associated with split eyes, which is where a normal colored eye will have a black/shadowed area, sometimes covering up half or more of the eye color. I used to have a king pigeon with one split eye (perfectly half black and half orange), and one plain orange eye. They make neat looking birds.
The colors of the eye is determined by the genes and amount of protien in the iris. Sometimes certain genes can modify the appearance of the eye color. So really all you can say about odd eyes is that it's just something that happens sometimes