Correct me if I'm wrong.

Healers generate enmity on 50% efficiency, and it's shared across multiple mobs.
For example, regen ticks for 348, that's 174 enmity generated. If 3 mobs are pulled, that 174 enmity is shared so each would only have 58 enmity.

Tank generates enmity on 300% efficiency, so for tank to pull off a 58 enmity regen tick on 3 mobs, he would have to do 20 dmg to each. Now I'm not sure how much enmity flash generates, but could it be less than 58 enmity? Now I'm sure as heck that shield lob doesn't do 19 damage at level 50...