From someone who does this sort of thing I can say the art director wanted the hands placed differently after he had drawn/colored the piece. So what the artist did was create a new layer and just tweak the hand but maybe forgot to hide the underlying area or mask out any extra elements.
Actually its called selective breeding, the 6 finger gene was slowly bred out of the gene pool. In fact a large number of 6 fingered individuals are still found in tight Amish communities due to the fact that they are so closely related