I did some tests on PTS to see how much bang for your buck you get from the Piercing star. So I went on PTS and killed a few VR10 mobs in The Rift with no points in Piercing. These are the numbers I got from a fully charged heavy 2H attack:

Wormcult Pyromancer = 4270
Wormcult Assassin = 3957
Bear = 3322

Judging by the variations in damage, it looks like Pyromancers are mitigating the least physical damage and bears are mitigating the most. Bears don't wear armor, but maybe they have "tough skin" or something. Not a big deal.

I then put points into Piercing to ignore 5% of armor. These were numbers I got from a fully charged heavy 2H attack:

Wormcult Pyromancer = 4310
Wormcult Assassin = 3997
Bear = 3362

I started solving simultaneous equations to find the two unknowns (base damage before any mitigation and mob armor rating). For example, a base heavy attack damage of 5070 and armor of 10256 will explain the Wormcult Pyromancer numbers assuming mobs, like players, get 1% mitigation for every 650 armor:

5070 * (1 - 10256/650/100) = 4270

If we ignore 5% of 10256 armor, that lowers armor to 9743

5070 * (1 - 9743/650/100) = 4310

But there is no way to get that 5070 base damage to work in any of the other mob's equations! For example, the bear, assuming 5070 is base heavy attack damage:

5070 * (1 - Armor/650/100) = 3322, works out to an Armor of 22400. Ignore 5% of that gives you 21230 armor. Putting that in next equation:

5070 * (1 - 21230/650/100) = 3414, significantly higher than the 3362 I actually get after 5% Piercing.

Then I noticed that all of the mobs took exactly 40 extra damage after 5% in Piercing. That would lead me to believe they all have the same armor rating! And that might make some sense. These are just random overworld mobs and maybe they are all generic and have same stats. But if they all have the same armor rating, why are the heavy attacks doing vastly different damage to them!

Am I missing something here? Does mitigation work differently than I think it does?

Thanks!