Mobs wearing armor drop that armor 100% of the time

An example, if I kill a zombie pigman with armor they drop full armor.

Steps to reproduce

• Make sure your map is in creative mode. (This also affects the survival game play.)
• Make sure your game difficulty is on the highest setting.
• Spawn a zombie until one is wearing armor.
• Give yourself a sword.
• Switch to survival mode.
• Kill the zombie that's wearing armor.

You will get all the armor the zombie was wearing. Repeat these steps and you will get even more sets of armor.