Put the summary of the bug you're having here

What i expected to happen is a diamond chest plate that was enchanted with thorns 2 and protection 4 to last longer than any other diamond armor i had, since the durability is so much higher

What actually happened was...:
had the inventory screen up and also had the durability monitor up for armor. Armor that has the enchantment thorns of any kind, IF it affects the enemy with damage it drops the durability of the armor twice per hit rather than once. So a chest piece of diamond decayed completely before any of the others remotely were close to breaking even though it's durability is the highest of them all.

Steps to Reproduce:
1. get diamond armor, enchant a chest plate with thorns and turn on durabiliy monitor for items
2. go into creative and get some golden apples, use them to have high health regen and turn to survival
3. Watch enemies beat the crap out of you and notice that the durability of the chest piece drops by a factor of two after every hit by going to the inventory screen and scrolling over the chest piece while being hit.

Code analysis by marcono1234 in this comment