Zombies and skeletons have a percent chance of wearing armor on higher difficulties, but the odds do not increase as expected. Currently they will have a 0% chance on easy, 5% chance on normal, and a 2% chance on hard. I don't know what it's intended to be, but it seems unlikely that it was meant to go up and then down.
Compare this with the following difficulty changes (easy, normal, hard):
Weapon enchantment probability: 0%, 10%, 20%
Armor enchantment probability: 0%, 25%, 50%
Pick up loot probability: 10%, 15%, 45%
Probability of additional armor pieces: 75%, 75%, 90%
Equipment enchantment levels: 5-10, 5-15, 5-20
Odds of a zombie carrying a shovel or sword: 1%, 1%, 5%
This would be super easy to fix, just one number in one file needs to be changed.