From the factory, the rear fenders are the same width 1973 up to 1979. The bumper mounts, as pointed out by Brian, are unique for 1973. A bumper shock was used in 1974+ beetles, and thus the fenders ended up with a D-shaped cutout for the bumper mount. 1973 fenders can be modified by cutting the D, while 1974+ fenders can be modified for a '73 by simply welding in a patch.
Aftermarket fenders, on the otherhand, are notorious for having wandering width measurements. Chances are you simply have two different fenders on the back of the bug. Possible that you have a Genuine fender on one side, and Aftermarket on the other. Or, you have aftermarket A on one side, and aftermarket B on the other.
-Dave
P.S. Front fenders are a totally different story. From the factory '71-72 have a different width than 73+, and european fenders are different then N/A.