Not every unit can have the same number of armor upgrades. The number each can have is determined by the files. Most can have 1, some can have 2, and a very few can have 3. It can be a rather drastic difference, too. Spanish Tercio Pikemen can have 2 armor upgrades. The second should put them in half-plate for an armor rating of 6; it actually gives them a 2. But as Musashi said, people found the line that shows what the armor should be with the upgrades, and it's commented out. Removing the semicolon in front of the line causes the game to crash. I don't know if it's an unfinished feature or something they just couldn't get working and left in the files.