i think it might have something to do with the metal. iirc my fathers 95 didn't show signs of surface rust on his rear calipers but both our 99s do.
also if you think about it the rear of the car is dragged around through everything, so it gets what is thrown up by the rest of the car
this is the perfect time to paint em. get some brake cleaner and clean and wire brush everythign up well then you can either tape everything off and spray paint or you can use a brush and paint the calipers. either use a high temp paint if there is one you like, like black is easy to find this way, or you can use a high temp clear coat like I did since I painted mine T-red
good luck and you should definately like the results.