Decay

The decay of your armor (how much it gets damaged) depends on what kind of damage it protects against and what kind of damage your receive. It also depends on the actual durability of the armor which can be found in the armor's statistics. For example, if you are hunting a Snablesnot (which deals Acid damage) while wearing Pixie armor (no acid protection) you won't get any decay on the armor, but you will receive the full damage of the hit. If you hunt a Berycled (which deals Impact damage) while wearing Pixie armor (Impact protection of 9), you will get decay on your armor, but you will also receive 9 damage point less per hit.

Most mobs deal more than one damage type, and if your armor protects against them you have the decay for that protection too. For example, if you are attacking an Atrox (which deals 33 Impact, 33 Cut and 33 Stab damage or more, depending on the size of the Atrox) while wearing Pixie armor (which has 9 Impact, 2 Cut protection), you will have decay for Impact and Cut damage. Your total protection against the 99 damage attack is therefore 11. It's a really stupid idea attacking an Atrox with Pixie armor by the way, since you will receive 88 damage on average each hit.