As I understand it, you only get the bonus from the single highest source. So, if you have something that gives you +1, +2, +1, and +3, you'll only get the +3 bonus. In your case, it looks like you're getting the +3 bonus from the Aura of Protection, but once that goes away, you'd be getting the +2 from your Constitution. I'm not 100% sure if that's the way it works, but either way it's overly confusing. The way they show it in the UI, it makes it look like it should clearly stack. I'm also not really sure what the point of having those bonuses on gear is. I guess you can pad the stats that you have 0 in? Or perhaps some gear will give lager bonuses in the later parts of the game that will be greater than the bonuses your own stats give? Not sure, but it's very counterintuitive if you consider how most games like this are played.