Ok so this is the problem, i replaced my brakes / rotors about 2 weeks ago and the car have been sitting in my driveway since then, so with the car been sitting so long there is rust on the rotors which is nothing; i took the car for a drive today and my back left driver side rotor still had rust in it while the other 3 tires rust was clean off when i pressed on the brake.

So now i want to know why is does the back left driver side rotor still have rust on it

More info- when i changed the brakes the back left driver side caliper wasn't frozen.