Is there any historical basis to units' attack and defense bonuses?

I'd add appearance too.

The unit I had in mind when creating this thread was the Teutonic Knight. He doesn't look dressed for combat, yet he's one of the slowest units in the game. Somehow, he's also incredibly resistant to melee damage despite his model not brandishing any armor like the militia line. I'd have also thought Knights would have horses, why do TKs don't?