When getting killed by anything that does durability damage to your armor (ex. Cactus, Mobs, Lava), the last hit that kills you doesn't damage the armor.

A reproduce method to follow:

1. Make sure the difficulty is on hard.
2. Equip some unenchanted armor.
3. Summon a creeper with /summon Creeper (this is so that it does the most damage possible)
4. Let the creeper blow you up and kill you
5. Pick up your armor back and you'll see that the durability is still undamaged despite the creeper killing you.

The only workaround of this issue I know if is to have a totem of undying on either hand, and let a creeper kill you as it still does damage to the armor but you haven't died yet since the totem of undying saved you from dying.

There are two videos to demonstrate the issue related to the description.