Negative Damage - Noob question

[cool_hc]

I was wondering what the mechanics were for negative damage from spells.
In many of my battles, I have noticed against certain enemies, whenever I have scored a hit with a battlefield spell, the damage comes up to be negative, which adds to the creatures HP, instead of deducting it.

Some specific examples, was when I was MA Shinuyama, I was taking on a bunch of MA Machaka Black Hunter spiders. I scored several hits with both Fireball and Shadowbolt, but it caused negative damage, leading to the Black Hunters HP shooting upwards.
I checked the Black Hunters specs, they did not have any elemental resists, nor did they have 'luck' cast on them. They were completely unbuffed.

So how does this happen?

The province the battle occurred in, did have Luck 3 though. Is this a factor?