Even assuming that there are only one of each, based on the few minutes of footage that we've actually seen, it could be something as simple as the same reason Drizzt has purple eyes when the rest of his race has red: because the author wanted them to. Or it could be something as simple as why there are white lions: it's simply a rare trait of the species.
Can you explain why a black person needs to justify their presence in a piece of media?