Ravenous Hunter/Gunblade. The Same but different?

Gunblade: Unique: Heal 15% of all damage dealt. This is 33% as effective for Area of Effect damage.

Ravenous Hunter: Heal for 2.5% (+ 2.5% per Bounty Hunter stack) of the damage dealt by your abilities. This is 1 third as effective on area of effect damage.

Ok so I'll start by saying I understand the difference between ALL damage, and damage done by your abilities. I'll also save people the trouble of pointing out you need to stack it before you gain 15%.

Right, with that out of the way, why doesn't ravenous hunt, FULLY stacked, heal as much as Gunblade even on single target abilities?

This came up while playing my electrocute Malz, I took revenous hunter and got it stacked quite quickly (8 minutes 42 seconds) thanks to some nice roams and a bad gank from their jg. at the end of the game I found i'd healed a megre 4.5k in 35 minutes.

I thought it was odd so went into the next game with inginious hunter, built my normal first 3 (rylais, liandries, serephs) and then a gunblade. In the remaining 7 minutes of the game I healed 13k damage.

I've tried this out with some friends in a custom using malzahar. I press E with gunblade, and i press e with a fully stacked ravenous hunter. In theory the fact I killed them all to get the full stacks should mean that ravenous hunter comes out on top because of the healing gained while stacking it, or at least even if you can't heal when you've taken no damage. Instead what we saw was gunblade healed for a three times more than revenous hunter did off one Malefic Visions. Also please note that Ravenous Hunter does apply healing from on hit effects like Liandry's, again I checked for this.

Can someone please explain this?