Possibly whomever painted the rear bumper didn't shoot it thoroughly...enough coverage.
The GFX paint will (or should) be the same specs as the car. Maybe they did a better job at shooting these than the factory?
It's got a cop motor, a 440 cubic inch plant, it's got cop tires, cop suspension, cop shocks. It's a model made before catalytic converters so it'll run good on regular gas. What do you say, is it the new Bluesmobile or what?