dirt, dust is causing the scratches. It gets trapped between the bumper and the iPhone and the handling of the phone moves the dust and dirt bits around causing the scratches.
you could try to buff out the frame, but the best way to do it is to remove the band and use a buffer. may not be practical as you'll have to deal with possible death of the phone if you screw up the assembly.
me, if the phone is going to stay in the case/bumper - out of sight, out of mind.