I read on some forum that elemental weapons ignore armor, but I didn't find any mention of that in the game manual. It didn't look that way to me, but then again the mobs I was fighting could have had elemental resists. Is there a way to determine if that's true?
Elemental weapons have two types of damage, physical and elemental.
If the attack connects, the target sustains full elemental damage, unless it has elemental resistance or immunity.
The physical damage part is a function of the target's armor and base damage resistance.
So, in your case, I think the mobs did have some kind of elemental resistance.