When a player is wearing armor, damage dealt by weapons is not calculated correctly.
Iron swords do not consistently deal the same damage on each hit on a player with iron armor, with it instead varying on each hit.
This does not apply to just iron swords and iron armor though, but it is the most noticeable there.

Steps to reproduce:

1. Hit a player, who is wearing iron armor, with an iron sword.
2. Check the players health after each hit, on hit 1 he should be dealt 1.5 hearts of damage, hits 2, 3 and 4 each deal 2 hearts just for it to revert back to 1.5 on hit 5.

For one thing: the damage should be consistently the same on each hit, it isn't

For the other thing: If each armor point is equal to 4% of damage resistance, then the 2 damage the sword deals are wrong. As it makes iron armor have 60% damage resistance, 60% of 7 isn't equivalent to 2 hearts however (4 damage points), its equivalent to 2.8 damage points, so 1.5 hearts. (1.4 hearts to be exact, but I assume the game would round that up)

A similar "phenomenon" can be observed when using any weapon on any armor stronger than leather.