Mine are the same, the rear wheels always get dirty with brake dust before the fronts. I think its something to do with the hanbrake mechanism being built into the rear caliper, there is quite a strong spring in the rear caliper for handbrake operation which keeps pressure on the piston and therefore the pads also if the handbrake operating lever mechanism shaft within the caliper is sticking this will keep a bit of pressure on the rear pads, i'm planning to strip and recondition my rear calipers this winter i'll see what I find.

Steve