Defense Stat System Broken?! :(

A friend of mine who plays with me purchased an armor chest piece in game which had 100 more defense than the previous piece of armor he was wearing.

 

When we went and fought mobs, the mobs attacked him for exactly the same damage with either armor piece. 

 

Is the defensive stat broken in the game or am I missing something? 

 

Thanks for any help you can provide!!

It could be you wearing an armor that not meant for your class. I.e a rogue (light armor) wearing heavy armor for esquire will have penalty on evade, move speed, etc