Frail Kiwi vs Multi-Hit Dual Gun Skills

I shot it with Bullet Slide, Bullet Storm, and Shooting Rush (1 damage x 4 hits), its HP bar barely moved as if it only took 1 damage, rather than moving past half of its health, and my normal shots do twice more than the above skills.

Glitch? Or skill mechanic?

It might have to do with the Frail's kiwi's special def/protection. This doesn't happen with pummel does it? if it's just dual guns, it's the skill that's the problem.

It's the skill. Pummel should work the same.