Defense - what is wrong here?

I have +316 defense from equipment and a total def of 476 on my pally
http://armory.wow-europe.com/charact...trider&n=Fàtal

When I hover over the defense number in game or in the armory, I see Defense Rating 311 (+122 ). What it teh +122 ? my Equipment should be giving me +316

1 Defense rating isn't 1 defense skill.

2.36 Defense Rating = 1 Defense Skill

Tanking Formulas:

http://www.tankingtips.com/tanking-formulas