Cheep discs can cause this hard /soft spots ,then you get uneven wear in the pads which accelerate the wear in the discs then you fit new pads and they ride on the high spots and the low spots get rusty because the pads aren't touching the low spots and so it continues until you fit new discs and pads and it all starts over again
When I was spannering back in the 80s you could buy abrasive pads which cleaned the discs up a treat don't seem to be able to buy them now
