[Dr3am]

I'm just guessing but part of it is most likely related to them being able to wear plate gear which has better overall tanking stats aside from the AC? I also read somewhere that wearing a shield caused the characters AC soft cap to go up and all armor is then considered under the soft cap. That won't matter for 2h wearing Knights, so my second theory is:

Ranger caps for defensive skills are low compared to the Knights (according to the wiki).

-Defense capped at 200 (according to the wiki) means they stop growing defense skill at level 40. Pally and SK cap at 200 until they get 51 and then it opens up.
-Dodge, Parry, Riposte are all lower than the Knight classes.

Early Tolans raises your AC value as you mentioned, but unless you are talking BP and Legs they are straight AC. Pally and SK can gear so many pieces that give them lots of STR (to block better with) and and STA. Just my theory though.