I had a bumper on mine and went about nine months without a scratch. One month I got lazy and didn't take the bumper off every few days to clean the dust out and that's how it got scratched. Dust got caught between the bumper and the phone and caused the scratches.
I still have the brand new in box apple free bumper that everyone got last summer and without a doubt it would end up scratching as well.
In all previous phone models, I always had cases on them as well and no matter what case I used, the chrome bezels would always scratch as well, probably due to dust and dirt as well.
Pockets aren't clean no matter how much we wash our clothes. There is dust in the air everywhere.