I think the defense stats fail to reflect the weapon reach advantage units with polearms or pikes have. For example: the French Vougle unit has crappy defense stats, yet only 1 unit of them can block a town/castle gate for a really long time (even the militia version which has about 25% weaker stats iirc), as opposed to the spearmen which actually have better stats.
I imagine it's the same with Varangian/JHI, and even makes dismounted noble knights a lot better than dismounted feudal knights, at least that's my experience in my french campaign.

This seems a side effect of the updated engine and battle mechanics: pike/polearm units will be very good at holding enemies off while in formation, but once soldiers get up close and personal they suffer from their weaker stats, as well as being vulnerable to missile fire. CA probably needs a new defence stat besides armour/shield/skill, or add the reach value to the skill stat to update the actual effectiveness of these units.

My little theory after noticing the values don't always reflect the actual combat effectiveness of units.