Damage, armor, and enemies

I want to make sure I am not missing something in how damage is calculated, so I'm asking the experts for some help. Not sure if it is a bug, or me just not understanding how damage is calculated.

While doing quests in the Charr lands, I was in the Siege Devourer. I noticed my damage was FAR less than what the skill said. Figured armor might reduce it some, but couldn't believe it got dropped as far as it did.

Siege Devourer Swipe says: "All nearby foes are struck for 100 piercing damage and are knocked down."
http://gw.gamewikis.org/wiki/Siege_Devourer_Swipe

Yet when I use this, I get numbers well below 100. I know armor will reduce that, but these are the numbers I got, and what enemies I got them against. Tell me how those numbers come out the way they do please.

Charr Axemaster
Charr Blademaster
Charr Bladestorm

Those 3 are all level 20 Warrior/Elementalists. I do 38 damage to them.

Charr Seeker

Level 20 Ranger/Elementalist. I do 84 damage to them.

Tumbled Elemental

Level 24 Warrior. I do 41 damage to them.

Grawl Champion

Level 24 Warrior. I do 41 damage to them.

Now, the Grawl and Tumbled are both Hammer warriors, so they do not have a shield to boost armor. They are also both higher levels than the Charr, yet they take more damage. The Charr Menders were not there all the time, and even if they were using Protective Spirit, there is only 1 Mender per group, and I would do the same damage to 2-3 warriors at one time. The can't get PS on all 3. The Wardkeepers were not around, so Ward Against Harm was not adding to the armor. None of the 5 types of warriors use armor boost skills of any kind.

Why was my damage less than 50% against those warriors, and reduced 16% against a Ranger? I wish now I had checked the damage against the caster classes, but never did.

Please, explain why these numbers are so low, or tell me I have found a bug.

Charr Seekers are lvl 20 rangers: they have 3*20 + 10 base AL, or 70 AL.
100*2^((60-70)/40) = 84.08.

Grawl Champions and Tumbled Elementals are lvl 24 hammer warriors: they have 3*24 + 20 base AL, or 92 AL. They also get an additional 20 AL vs physical for a total of 112 AL.
100*2^((60-112)/40) = 40.61.

Charr Axemasters, Charr Blademasters, and Charr Bladestorms are lvl 20 sword/axe warriors: they have 3*20 + 20 base AL, or 80 AL. They also get an additional 20 AL vs physical, plus 16 AL from a shield, for a total of 116 AL.
100*2^((60-116)/40) = 37.89.

[wiki]Damage_calculation[/wiki]

Monster armor levels are generally the same as hero armor levels: [wiki]Hero#Armor[/wiki]

Wow. Thank you. I new armor made a big change in damage, but didn't realize such a small increase in armor made such a big difference. And yes, I consider the armor changes from the 2 types of warriors small, and the damage reduction big. +4 armor is a -4 damage.... surprises me.

Just for the above equation only. Different base damages will change the amount armor reduces.