There has GOT to be a bug somewhere in damage calculations

I really don't understand how I can go through maps with no Extra Damage mods and some seem to just randomly hit me like a truck, go through other maps WITH an Extra Damage mod and not notice, and then other times an Extra Damage mod will result in a single rare one-shooting over 3200 health at 78/76/78/57 and 70% armor.

Anybody else feeling like there is something somewhere completely out of whack? Because it is just making zero sense. Had an Extra Damage mod with the meteor spell casters and it didn't hurt any more than it tends to hurt without it. I've had a rare mob hit me for <200 a swing only to encounter another in the same map of the same mob type hit me for over 2500 a swing. While some of this experience could simply be map/mob level differences (weird that only one or two levels difference would mean nearly doubling mob damage in some cases I've experienced) it makes no sense how many times I run through some T13/14/15 map where a level 80 map won't even raise my heart rate while a 78 I'll have a one-shot situation, or an Extra Damage mod won't even be noticed in one while another I'll have three white mobs do 2000 damage to me leaving me just jaw dropped in confusion. It's like certain attacks can hit twice or something, or have a misplaced decimal, or randomly always crit, or have a penetration that triggers more than it should or is waaay too high. I just don't get it and it only makes so many of my deaths that much more confusing and frustrating.