Area of effect damage bug?

[Accaris]

I think I might have found a damage bug while running around with a level 20 Elementalist/Monk killing minotaurs in the Northern Shiverpeaks. I'd let a group of five or so monsters start hitting, and then use Flame Burst, which deals 113 damage and drops their HP by about half. I wait for Flame Burst to recharge and then do it again. However, the second time the enemies' health bars hardly move at all, even though the damage dealt was the same! I discovered that this phenomenon occurs with other AoE spells like Fireball and Inferno, and also with different enemies (like the Dwarven Axe Fiends, the Charr, and Mergoyles in Kryta.) What gives?

[Mariers Arnimane]

I'm not exactly sure what skills those minotaurs use, but I'll bet it's something like Frenzy, which will make them take about 33% more damage. After Frenzy is over, they go back to normal (plus resistance, if they have any), and you do less damage.

[Mkillerh]

Frenzy is double dmg

And if they dont have frenzy, its endure pain maybe...