Funny stuff.

Possibilities:

1. It is painted
2. It is fake
3. The surface is processed in some way, but not necessarily painted.
4. one and two
5. Chemical contamination.
6. Bad batches has found their way to the market (more or less 2)

IMO number 4 is most reasonable. I mean it is well known that the N9 is not sold in US, UK and so on, yet the demand is large. Lots of "original replacement parts" can be found on the net. For outlets it is not particularly difficult to mix in non-original parts, and the risk is close to nothing. The internals are original (most of it), but the housing, buttons and so on can be fake.

Number 3 is also plausible. Surface processing is a normal procedure for outdoor optical PC lenses (car head lights mostly). They will seldom brake or crack, but they are hardened to prevent scratches. Gorilla glass - same stuff. This can be combined with 6. This kind of surface processing may also include matting of certain areas for wanted optical effects.

The least speculative, the most reasonable explanation is 4. Therefore it most like is the truth.