Would like some clarity on wording as well... can't remember which trait I was considering, but it was basically +3 where the previous trait was +2, so I was there going "wait, do I get +5 total or just +3?"

It should be stacking (not replacing - that just isn't how traits work). You can see this particularly well if you take Dodge I-III and check your dodge value, or in the case of Precision, check your accuracy. I do agree that it needs better feedback because some shows it better than others. Perhaps what's really needed is just better tooltips.