One out there possibility is called chimeraism. Basically two separate fertilized eggs fuse together to make one offspring, sort of the opposite of identical twins. Parts of the animal are from one of the eggs and other parts from the other egg. No telling which is responsible for the reproductive tract.

Chimeraism has recently been recognized to happen with humans after investigating apparently impossible paternity/maternity tests but it's unclear how common it is. If it happened with normal ball pythons or even in clutches that could only produce the same mutation you wouldn't notice it. It's only when a clutch could produce more than one type that you have the chance for say a mutant and a normal to fuse. I've not yet heard of one of these paradox albinos coming from a clutch that couldn't have produced both normals and albinos to disprove the theory but that's not saying much as few albino X albino breedings are done and most do involve hets.

Even if chimeraism is responsible for some of the paradox animals there could be other causes seen too.