It is quite normal for some Saddlebacks in a litter to be mis- marked. It can vary a lot across a litter from perfect to all black, all white, or almost anything in between. Obviously any pig that doesn't have the correct pattern cannot be registered in the herd book but it won't affect the taste of the meat. Also bear in mind that the pig's markings are just part of what constitutes the breed standard and there are other things to be taken into account as well.