That happens because it's a different material. I used to work for a tier 1 company here in the Detroit area - the company used to be Creative Industries, was bought out by MascoTech Special Vehicles, then turned into MSX International in the mid-90's. I worked there from 1994 to late 1997. We painted ALL the first gen Vipers, & all the Prowlers. We were a HUGE paint operation. We also built some concept vehicles for Ford. We had huge programs with both Chrysler & GM - we would paint a lot of their show cars. I remember a Cadillac STS or similar model with a new candy red color for the upcoming year. Paint department painted the entire car, including the fascias out of the same batch, same day, same booth. When the car was assembled, the fascias were WAAAAAY off - they did not match at all. Thus began a long series of custom-blended versions of that color until they finally got it almost the same as the rest of the steel body! It was pretty unbelievable what they went through to get a match. Think about it - the paint was baked in our booth, & the plastic of the fascias reacted very differently than the steel body - and that affected the curing & characteristics of the paint.