[Multiplayer?] Player hits will sometimes increase by a very high amount, such as hitting with bare hands may rarely deal 3 hearts

I was playing at Jesper the End's server, on the arena minigame, and I found out that for some reason one of the players was dealing 3 hearts per hit with his bare hands. I first thought he was hacking, but after I talked to him I understood that its a bug.
(Note: No, this isn't a bukkit server, either a modded server. This is a vanilla command block-made minigame server)

What I expected to happen was...:
The player will deal half a heart, or 1.25 hearts, if he did a crit.

What actually happened was...:
he always dealt exactly 3 full hearts (Making it so 3 hits will make the player on one heart, then another hit will completely kill him.)

Steps to Reproduce:
1. Join a PvP enabled server or a minigame server with a battling place.
2. Attack a player with close combat (hands/swords) for quite a while (Might be better to find more and more players until one is found)
3. See that the player is dealing quite a bit more damage than what he's supposed to do (The most easily shown way is no armor and bare hands fight)