Its as many posted above - genetics. However assuming that a misbar is a recessive trait (which is pretty safe since the "normal" clown doesn't have them), you'd have better luck breeding misbars with each other. This isn't to say you won't get misbars from two "normal" clowns, you will. Your chance at misbars would just be higher is all. Some clowns just don't misbar that often (clarkii come to mind), where others (Maroons in particular) seem to have a pretty high number of them.
Check out Joyce Wilkinson's book "Clownfish" for everything and more you'd want to know about breeding these guys!
Good luck.