There's usually a slight different color between the the front/rear bumpers and body in most car. It's because the bumpers and the body panels is made of different material. Even if you prep and use the same paint code on it. The color will never be exactly the same but it's hardly noticeable.
It used to be noticable from GM vehicles. Not sure how true it was but I was told if there was an exact match in paint ( body panel vs plastic bumpers) it was a sign that the car was repainted (accident?).
It does happen that during manufacturing the car color will get messed up and won't be completely the same across the whole vehicle, but I bet you that it just has to do with the difference in materials.