I am not 100% sure on this, but it is probably that a +X bonus from a trait is considered a +X trait bonus, which means they would not stack (such as how enhancement bonuses and morale bonuses do not).
Edit: I took so long to hit post that I got ninja's. I also found the rules excerpt:
"Many traits grant a new type of bonus: a “trait” bonus. Trait bonuses do not stack—they’re intended to give player characters a slight edge, not a secret backdoor way to focus all of a character’s traits on one type of bonus and thus gain an unseemly advantage. It’s certainly possible, for example, that somewhere down the line, a “Courageous” trait might be on the list of dwarf race traits, but just because this trait is on both the dwarf race traits list and the basic combat traits list doesn’t mean you’re any more brave if you choose both versions than if you choose only one."