Mine are the same and I am 99.99% sure they are made this way.
The 'polished' finish on diamond cut wheels is achieved on a diamond cutting lathe. As part of the manufacturing process, the wheels will initially be completely powder coated black. The face will then be cut on a diamond cutting lathe to remove the black powder coat, creating the polished face, and leave the rest of the wheel (recessed parts and reverse side of wheel) black prior to the lacquer coat being applied to the wheel face.
So the 'rings' and the slight ridges that the OP can see and feel will be the result of diamond cutting process, as the diamond tipped cutting tool moves across the face of the wheel on the lathe.