Damage taken by Zombies appears to be less than it should be.

Easiest way to test this is with a smite 5 diamond axe. In vanilla hitting a zombie would kill it in one hit, but in spigot it would take 2 hits.

In my testing, in 1.9.4, I used the client-side mod ToroHealth Damage Indicators to show the health of the mob after each hit. (If the mob died, then I could only state the maximum hearth of the mob as the damage done):

spigot

diamond sword:
Zombie: 13.56 left - 6.44 damage
diamond axe:
Zombie: 11.72 left - 8.28 damage
diamond axe w/ smite 5:
Zombie: 0.22 left - 19.78 damage

vanilla

diamond sword:
Zombie: 13.11 left - 6.89 damage
diamond axe:
Zombie: 11.14 left - 8.86 damage
diamond axe w/ smite 5:
Zombie: 0.00 left - 20.00 damage

In 1.10 that mod does not work, so I have instead used two plugins to show the damage, CombatIndicator(https://www.spigotmc.org/resources/combatindicator.5834/) and RPGHealthIndicator(https://www.spigotmc.org/resources/rpg-health-indicator.21185/):

Diamond Axe w/ Smite 5: 0.22 left, shown to do 21.5 damage

for comparison, a Diamond Axe w/ Smite 5 vs Skeleton: 0 left, show to do 21.5 damage

Images attached of tests in 1.10 spigot.