Was the enemy you attacked allready damaged by a hit you dealt before? I observed something similar with leech a while ago and I think, that you don't heal from "overkill" damage. If the enemy you are attacking has only 1000 hp left, you will heal only for 300, even if the damage is shown as 20k.

I'm not 100% sure about it, it was just my conclusion I made from my ingame observations. Maybe you can help to confirm or refute.

This, it works correctly.  The damage is the "strength" of the hit, but if the enemey only has a little XP left you aren't doing any damage...

Yep. It is working correctly. You only heal 30% of the life you "steal" in the lifesteal set. if there's very little life remaining, there is very little to steal.

Thanks a lot everyone, this makes perfect sense now.

Alex usually open with Spraydown, so the enemies have full health and I get full heals.

Next he use Born to Win, but on an enemy with 10 - 20% health, so its a small heal

Lastly he kills the rest with Artic warfare, wich gives small heals, but the next wave Spraydown and Born to win are still on cooldown, so he gets full healed on those damage numbers. 

I always just focused on the damage numbers displayed and didn't take into consideration the enemy's health. Obviously the "damage dealt" as stated in the set's description cant be more than the health the enemy has left.

Also note that damage to shields doesn't count, IE you don't heal off of it. Likewise if they are running block damage, or if they are unkillable but with 1 hp, you won't heal either.