There are many reasons why this happens. It happens at the factory and at a collision shop.
1. A bumper cover is essentially a panel paint. You're painting a part with a large portion of it being flat against another panel. Blending will cure this but it's not going to happen at the factory and rarely at a body shop.
2. Cars are painted at the factory where the bumper covers are painted off site. Lots of room for difference here.
3. The evaporation rate is different from metal to plastic. The slower the solvent evaporates the longer the flake has to float instead of stand up. This will change the face of the color.
