I had the front brakes on my '84 T-Bird serviced today. When the mechanics removed the wheels, I saw that the right-hand rotor–which I had already suspected was worn–was deeply scored. The scores looked much like deep grooves cut into an old phonograph record. Why would this happen to one rotor and not to the other (all they had to do with the left-hand rotor was “turn” it)?
It reminds me of an old joke: You wear shoes on both feet, and use both feet equally, but you get a hole in the sole on the left shoe of one pair and in the sole of the right shoe of another pair. 
something pitiful…
Did you still have some meat on the pads? If you did, then sometimes crud, gravel, etc. gets caught up between the pad and the rotor. Also, the quality of the pads also comes into play.
If there was no meat on the pads, well, then, there’s your answer. 
Also, on older cars especially, the actuating-piston pressures in the two wheels aren’t necessarily the same. This leads to differential wear on the pads and if one gets worn down to metal the rotor will be scored.
What did the inside of the rotor look like? My old pickup truck did this and it turned out the reason why was that one of the pins that the caliper is supposed to slide on had rusted into place and wasn’t sliding any more, which made the disk pad wear unevenly and gouge the rotor long before the other side wore out.
Uneven brake wear usually means a problem with a caliper (sticking piston or sticking slides) or a problem with a brake hose. However, a small degree of unevenness is not unusual – if the pads on one side wore down to their metal backing and on the other side are very nearly that worn, there might not be a further problem.
There’s also the possibility that this happened with a previous owner of the car who opted to not replace the scored caliper. If the pads on the side with the scoring are not yet down to their metal backing, this could be the case.
Another thought – given a equal amount of time turning left and turning right while driving, the right side could wear more because left turns in the U.S. are wider than right turns, putting some more miles on the right tires and brakes.
I didn’t see the rotor after they removed it, except once on the ground, face up; they don’t like customers entering the work area.
The car has been in the same family since late 1985. I’ve had the brakes serviced at a local Firestone several times since it’s been in my name, so maybe a mechanic there screwed it up. (One of their people was so clueless he tried to use the air wrench to unscrew the nuts on the decorative hubcap! Pried three dummy lug nuts off that way. I wish I’d had the presence of mind to complain about it then.)