From looking at the pictures, I'll bet a new spelling checker that the
piston on the side where all that rust is evident is frozen in the cylinder.
If your calipers only activate from one side with the other side a passive
pad the disk has to be pushed over to contact the other pad and if you push
it over it bends from the centerline and contacts the outer part of the disk
first.  Looks suspiciously like that is the case.  Note the wear marks on
the outide portion rather than the inside.  Without the wiping action the
surface is never cleaned and corrosion like compost occurs.

Mike 'em get em turned mike em again.  Then replace the caliper on one side
with a known good working caliper.  Chalk the disk and see if you get
contact over the full swept area.