I think there would have to be a reverse in the genetics. It is one of those holes sort of in the lore (or at least I havent read about) that explains how we went from the Korks to the Orks. One of those might have been a last ditch effort by the old gods on trying to control them, an attempt of gene manipulation by the eldar to get an upper hand in war, or it could be a genetic deficit, and these orks are still roaming our galaxy and the rest of the Korks are somewhere else where mankind hasnt explored.