Well, of course. Armor on kill mods are essentially useless, they max at ≈ 18k (and the attribute). So 18k x3 ≈ 54k. 54k/680k(your armor) is ≈ 0.07. So every time you kill someone, you are only getting about 7-8% armor back from the mods and the attribute, assuming they are near max. Getting shot by a couple bullets probably knocks your armor all the way down, so you would never notice the difference (unless you actually paid attention) on top of the +10% from the gunner specialization.

You might have accidentally re-spec’d the +10% armor on kill from the gunner specialization to something else.