2013 white platinum metallic tri-coat here.
I've always noticed a slight difference in color from the bumpers and the body on my car. I even pointed it out to the salesman when I was shopping. We looked at several different models (mustang, fusion, etc.) and all of them had a minor color shade difference on the bumper and body, with the bumper usually a bit darker than the body. It is more noticeable on some colors like white platinum. Like others have pointed out, the difference is because the bumper is plastic.