Yep its down to the different materials and it is made worse by the fact that the bumpers/ car get sprayed with different batches of paint. It also gets worse as the car ages too because paint fades faster on metal surafces than plastic so your bumpers will appear darker with time.
I had this problem with my Alfa Mito, when I went to collect it after waiting 8 long months, I noticed it straight away and I refused to accept it until they sorted it out. After another week I went back and it had been sorted out. Funnily enough though they never painted it, they had another bumper plucked off the production line in Italy and shipped over. That bumper matched like a dream though, so again highlighting the different paint batches problems.
The "higher" end manufacturers compensate by spraying the plastic panels with a different shade of colour ( slight lighter) so that they match perfectly with the metal panels, I know this is the case at Jaguar at least, because I have seen it in action
I had this problem with my Alfa Mito, when I went to collect it after waiting 8 long months, I noticed it straight away and I refused to accept it until they sorted it out. After another week I went back and it had been sorted out. Funnily enough though they never painted it, they had another bumper plucked off the production line in Italy and shipped over. That bumper matched like a dream though, so again highlighting the different paint batches problems.
The "higher" end manufacturers compensate by spraying the plastic panels with a different shade of colour ( slight lighter) so that they match perfectly with the metal panels, I know this is the case at Jaguar at least, because I have seen it in action