I have a med 256SL with XTR M9000 with 160mm Ashima rotors and my pads wear evenly.
As 325racer said, remove spacer under the calipre mount if you have any. I'm guessing that's not the problem. In which case, this is a bizzare problem to have.
What size frame? Maybe there's a manurfacturing error on the caliper mount on your frame? Can you measure the distance between the calipre mount surface and axle center?
What axle are you running? 142x12mm thru or 135QR?
Some possible fixes:
Change dropout / RD hanger to move axle further from calpire mounting surface.
Contact iPlay for recommendations.
Chnage to 180mm rear rotor and add appropriate spacer under calipre.