Those mounting ledges are really rails that the brake pads ride on. They take all the force of the brake pads trying to spin with the rotor as they clamp the rotor. The caliper is a floating caliper, meaning its just mounted on its own sliding rails/pins to fit over the pads and squeeze them like a vice, the caliper doesn't have to fight spinning with the rotor, the pads are on rails to counter that.
As the pad wear and become thinner, they have to move along that rail closer to the rotor so they can still make contact with the rotor, even though they are thinner. The caliper, having only one piston, has to also slide along its own rail/pins as the single piston extends to take up the difference of the thinner pads and pull the pads. This set up is far cheaper and more reliable, then other set-ups where you have to have multiple pistons on each side of the caliper to equally squeeze and move the pads and the caliper has to take up all the force/torque of wanting to spin with the rotor as it squeezes the rotor. The floating caliper has 1 piston, and its able to slide around over the top of the pads, which the pads are mounted solid on the rails around the rotor."
Here are mine below. The top and bottom rails were worn like this on both sides of the Jeep...
When the rails are worn, your pads can't slide properly and get jammed and twisted causing low pedal, uneven pad wear etc..
So, i got the welder out, and with a little dab of weld and a bit of grind i got a nice smooth surface for the pads to run along.
Next job was to replace the pads. The new ones were so unevenly worn due to them getting jammed on the rails and twisting, that i chucked them even though they have only done 200 k's. The local auto shop had some cheapies for $39 and it was worth me grabbing them just to trial everything out. I also believe my calipers are knackered. They are 20 years old and a bit of wear here and there and even with the rebuild kits i put in, i think they are sticking, which is also wearing the pads.
Anyway, replaced all the pads, did a bit of bleeding and all of a sudden i had a nice pedal again. Just a small touch of the brakes and they are grabbing again, so there you go!!