Those are brake rotor hard spots.
Rotors can also develop hard spots that contribute to pedal pulsations and variations in thickness. Hard spots may be the result of poor-quality castings or can be caused by excessive heat that changes the metallurgy of the rotors. A sticky caliper or dragging brake may make the rotor run hot and increase the risk of hard spots forming. Hard spots can often be seen as discolored patches on the face of the rotor. Resurfacing the rotor is only a temporary fix because the hard spot usually extends well below the surface and usually returns as a pedal pulsation within a few thousand miles. That’s why most brake experts replace rotors that have developed hard spots.