hmmm maybe they just never bothered to seperate kangaskhan from the baby when it is born from the egg in the game and how long and complicated would it be if the kangaskhan was without the baby, and then when it had an egg, it would need to change to sprite to where it had the baby kangaskhan in the game
because the kangaskhan's was origionally ment to have a first level (baby in it's pouch) but last second changes from the pokemon company boss cause baby kang. to become the glitch known as "missingno." and they never changed kangaskhan's sprite. that is why "missingno." can sometimes evolve into kangaskhan if your game lives through the glitch.