Some mobs float above horses when riding them

I was trying to make a wither skeleton riding an armored horse.

When I spawned it in using commands, it was slightly flying above where it should be sitting.

Use this command to summon it

/summon Horse ~ ~ ~ {Passengers:[{id:"Wither_Skeleton"}]}

Affect passengers:

1. chicken

2. cow

3. donkey

4. enderman

5. endermite

6. horse

7. husk

8. llama

9. mooshroom

10. mule

11. ocelot

12. pig

13. polar bear

14. rabbit

15. sheep

16. silverfish

17. skeleton

18. skeleton horse

19. stray

20. vex

21. wither skeleton

22. wolf

23. zombie

24. zombie horse

25. zombie pigman

26. zombie villager