While you're right that this doesn't happen on live (unless maybe under some special circumstance such as affected by spells or racial modifiers), what Demonstar said still rings true.
EQEmu simply pushes an update to hp and mana every two seconds; This becomes an issue because the client and the server fight for different values at different intervals.
I've heard of a couple people solving this problem. I've never seen it fixed personally. If you reverse engineered the client you could probably figure out when to properly push updates. And yeah, it sucks. It's the one thing I wish was fixed
