I think in the specific case of the OP it is due to
scaling. I have been doing some number crunching (I am an engineer by training) since I made this
post. My goal with that post was to show people how much damage the Warmaster needs to be defeated so people could set damage dealt goals. Even though the overall premise was right, I think I came to the wrong conclusion.
I remember reading somewhere before the DLC 2 release that the Warmaster was a fight for 14-24 people and that the instance would scale (though I cannot find that source now). So I have been looking through youtube and the forums for people who actually displayed the damage numbers and I have been performing calculations on overall damage.
Preliminary results indicate that the magic number is about 1.15 Million damage per person per instance between 14-24 people (give or take 150K per person).
So for the example of 14 people or less, the amount of damage needed to defeat the Warmwaster is around 16.1 Million (plus or minus the error). Then finally scales to 17.3M for 15 people, 18.4M for 16 people etc.
Some variability in the damage results comes into play when people hit unintended targets (ie the wrong armor) during the fight, thus inflating damage numbers. Also, I have found out that outside damage does not play a role in your final damage numbers (a quick test is go kill a bunch of stuff outside, then do not kill anything inside and you will notice 0 kills to your name when the fight is over).
I am hoping to have more results so I can post a more firm estimate on damage in a little bit (so stay tuned...).
PS On another note, I too find that hitting the back is an easier (ie quicker) way to remove the armor and provides a larger wolfhound target especially for people with aiming issues. But I go with whatever people in that instance want.