14% is very close to 12.5%, which is what you get when one of your great-grandparents was black. Then it seems most likely the 1.5% difference is what actually is the statistical noise.
However, it seems weird that he wouldn't know that one of his great-grandparents is black. I mean, I don't know too much about my great-grandparents either, but if one his grandparents would be black, that's something you still can clearly note, except if they died early. And his parents never told him that one of his great-grandparents was black when he was developping his white supremacist ideology?
Not saying it's all impossible, it's just weird. Another possibility is ofcourse he had two black great-great-grandparents. Both would have added each about 6.25% genes, so together 12.5%. That seems more probable to me about him not knowing he has black people in his family tree.
Anyway, just some thoughts about the genetic test...