Help me understand this please.

I have a level 94 merc (from act 2 NM, with might) equiped with an eth Insight CV, andy helm socketed with a ral (non eth unfortunately), and an eth fortitute sacred armor.

Once chilling kicks in it's over 12k defense and does about 1525-8497 dmg, res is fire/cold 75%, light 80%, poison 85%.

Given all this why is he being melted within seconds sometimes? Most often it's in Trav, but when I went to cold plains to see how much defense he gets once chilling kicks in, I noticed his defense would sometimes drop down to barely over 1000 and fire/cold/light drop to 23%.

Am I missing something here? I don't recall this happening on my merc on last seasons ladder.