Ok so I was messing around with a diamond sword enchanted with smite 5 on my creative mode world and I noticed that it killed skeletons, phantoms, and other undead mobs in 1 hit, but it killed zombies in 2. Then I tried killing a skeleton with a wooden sword and it did it in 4 hits not 5.

How to reproduce:

1. /give @s diamond_sword
2. Enchant it with smite 5.
3. /time set night
4. /summon skeleton
5. Hit the skeleton with the diamond sword.
6. It should die instantly.
7. /summon zombie
8. Hit the zombie with it as well.
9. /summon phantom
10. The phantom dies in one hit, just like the skeleton.
11. /summon stray
12. The stray dies in one hit.
13. /summon wither_skeleton
14. The wither skeleton dies in 1 hit.

Obseverd results: The zombie dies in 2 hits, but the skeleton, stray, wither skeleton and phantom die in 1. 

Expected results: The skeleton, phantom, wither skeleton and stray die in 2 hits, just like the zombie.