Knights seem to have the same defence value. Gothic Knights have 15 defence as Imperial Knights and Feudal. The only difference is that the armor / defence skill is altered with later knights having greater armor and lesser defence skill. Does that have a difference apart from the armored units being vulnerable to armor piercing units?

I mean shouldn't later Knights have greater defence? Doesn't lowering their defence skill equates their upgradet armor? Or is armor value more significant than the defence skill?