I have a Canvas Cover that says it provides 2% Storm defense and the Raiment of Storms robe that says it provides 3% Storm defense. When I wear one or the other I get the amount of defense that particular item specifies yet when I have them both equipped it shows I only have 4% Storm defense instead of 5%. Now, it's been quite a since I was in school but if memory serves, 3+2=5. :P
So it seems there is either a bug in the way these are being added together or the way the total is being displayed. Or is there some kind of limit to what stacks?