But that's not how the game works it, you see.

For example, militiamen (with padded armour) have 3 defense from armour, and peasants have 0. Now, it's logical that upgrading peasants to padded armour would give them +3 defense to bring them equal to the militiamen, but it only gives +1.

Thus, you can be up to some form of mail in looks when the unit has only gained armour equivalent to padded leather.