A psychologist could read a lot into which pages you chose to color (right brain duck) and which you just stuck to the lettering (left brain elephant). I won't judge though, we'll just love Li'l Jamie and be thankful he has all 12 toes and 13 fingers.
It may have been due more to crayon availability ! One dinky box to serve 20+ kids. I once met a species of Cat that had extra thumbs; they evolved during the early days of America and were efficient Rat-killers aboard ships transporting goods from England.