Skeletons in Nether

I think its intended that skeletonspawneggs sometimes spawn regular skeletons as a possible way to spawn them fast inside the nether (even if i would prefer 2 spawneggs), but i think its a bug that skeletons spawn NATURALLY in the nether! they are mobs
from the overworld and i think it isnt expected that they spawn at netherstrongholds instead of witherskeletons!
TO REPRODUCE:
Go to a netherstronghold and walk around a bit and look at the skeletons hiding between the witherskeletons!