Is this a bug, or do they just have a ton of hp?

(55 H 14 M 44851X): You attempt to hide in the shadows.
You slip into the shadows unnoticed.
(55 H 14 M 44851X): You attempt to backstab the weathered old pirate.
You backstabbed the weathered old pirate for 54 damage.
(55 H 14 M 44851X): You kicked the weathered old pirate for 14 damage.
(55 H 14 M 44851X): The weathered old pirate missed you.
(55 H 14 M 44851X): You slashed the weathered old pirate for 22 damage.
(55 H 14 M 44851X): You see a weathered old pirate.
He looks to be in quite a foul mood. I wouldn't bother him.
He has some small wounds and bruises.
He looks very angry at you.
He is attacking you.
He is a little better than you.

This happens alot on a variety of mobs. Especially if you crit them right away, or bs them or bite them, they will usually tick full before your next attack. Whether it is a bug or not......

Well, if this isn't a bug, it rendars backstab completely useless.

kill em quicker

I don't know the mechanics behind it, but Huklebuc is right, it does happen to alot of mobs. It seems to occur on the ones that have been baited, and have been standing there a while. I've had gnomish minstrels at "nearly dead", and then on the next hit, not only do they survive, but they are in excellent condition.

If it's a big problem, start killing from the back to the front. That way you'll be attacking the fresher ones. Just let the older ones wander.

If it is a bug, it's going to be very hard to track down, especially if we dont know why certain mobs tick and other mobs dont.