It MAY take longer in the rear, as they are not as heavily engaged compared to fronts. If you are regularly in situations where traction control or AWD is called for, then yes, you might actually see them cleaning up FASTER than the fronts. This is due to a feature called Electronic Brake Distribution, part of the traction control features package.
As to how evenly the coating should come off and whether the pattern you posted looks normal, I have no experience. I do know that if you buy the coated Ford/Motorcraft rotors, the wear pattern on the coating looks similar, almost like grooving, but after the coating is completely gone, the rotors look just fine. So maybe just hang in there a bit longer. As long as brakes are working fine, really, no worries IMHO.