Damage... variance?

Describe the problem. Be as specific as possible!:
I was killing level 27 white baby monkeys for a quest, when I noticed that I was killing some of them with 200s and the others were taking 1000+ damage and not dying. A 900 supposedly did half their HP to one, and then it died from a 300. Upon closer inspection, it wasn't just White Baby Monkeys.

 Images of problem

Will get some when I can.

What class are you?
Bunch of classes have client-sided damage only.

This is what archers deal with alot. The first attack hitting shows the damage of what the next attack after will be.

so lets say you deal in order 400, 500, 500, 500, 800, 600.

400 really is 500, 500 is the second 500, the second 500 is the last 500, the last 500 is really 800, and the 800 is really 600. It just keeps going like that.

What class are you?
Bunch of classes have client-sided damage only.

I was playing as a warrior.

I was playing as a warrior.

What skill were you using?

Power Strike and Slash Blast, this was long before I had mastery, so I had a huge range.