Deathblow Doubled Gauntlets - Culling Strike

The Culling Strike on deathblow isn't working or the description isn't clear.

If I hit normal monsters, I keep getting monsters with 10-20% hp remaining. Based on the description this shouldn't be possible as any <30% hp monster should die instantly.

My guess the culling strike is the same as the gems one, that will apply only to Rare and Unique monsters. If this is the case, this is pretty bad :( as a Warrior there is no Culling skill available.