You're correct GURU, some late era units skip the padded leather upgrade, for example musketeers or tercio pikemen. It's no armour -> breast plate -> half plate for them.
To make things even more complicated, late era units get other upgrades than early units. This is to reflect technological advances that took place, but their armour value is the same. Breast plate is armour level 2 and half plate is level 4 - when no armour is level 0.
The battle model is broken for retinue longbows, too. They start with heavy mail, then go back to leather and light mail. You can repair this in the EDU by simply changing their order in the battle_model line.