I think its the lighting and angle the light hits the panel. I notice the same thing on WRX front fenders, I always think its a different color when I see them from certain angles

 

i have a silver 05 coupe and i felt the same way....i am still a little paranoid over it...but i figure if i have to look at the car that hard to see a difference then it is close enough....lol

 

It's common. My Honda Odyssey that I bought new is like that, and my G35 is also like that.
I have concluded that if everything is straight, just enjoy the car and forget those metallic anomalies as they will drive you crazy otherwise.

 

I just had some body work on my BS 03 coupe, I had the infiniti dealer do the work and they said it is a common problem with infiniti's that the paint they use does not match on the plastic areas vs the metal areas its always minimal but over time it will become more apparent

 

This.

Really though, go look at any other make/model car with plastic bumpers, they're quite often a slightly different shade when looking at silver (which is the hardest color to paint match to begin with).