Another (useless/FYI) paint rant. :)

TypeOne

Ask any body shop... anyone that has been painting any decent amount of time will tell you that there is a definitely a difference in what material you paint.

The metallic flake will lay differently on a metal surface compared to plastic, carbon, fiberglass..ect

I'm not saying that it happens in every single case where metallic flake is used, but I'm saying it happens. If you have the skills to work with it and the experience to know how to lay it so that the affect is reduced then you won't notice it... but otherwise you'll see it.

I've been involved with body work and paint shops for a long time and have painted a few things myself. I've had about 4 cars completely repainted inside and out.. I know because I'm super anal when it comes to paint.

In the case of a Porsche being perfect, I'm sure it is. You pay for it when you buy the car, but being the Z is a less expensive car made with less expensive materials this type of issue seems more likely.

To the OP, are you sure your bumper was never painted after the factory?

And as Cab83_750 said, painting parts in separate batches or even changing the direction of your spray pattern can cause flake to settle differently.

Even though you paint two parts with the same material at the same time, spray them differently and they can have two different affect when they dry.