Skeletons spawned with /summon or natural spawning algorithm are not in the correct height

Skeletons spawned natrually are in the same height with Creepers.
Skeletons spawned with eggs are in the correct height.

On the left is a skeleton spawned with the command /summon.
On the right is a skeleton spawned with egg.
The left one can be pushed out but the right one cannot.

I' m not English so my English isn' t very well