I have been finding shorter ground cables on 65 cars over the years and never paid much attention to it. Have had to shorten the reproduction wires in the past to get it right on show cars.
Recently went through my original ones and noticed that the short ones have an FDD-14303-A SX FOMOCO marking and are about 7.5" long, total length. The 66 version is marked FDA-14303-A SX FOMOCO and has a total length about 10 1/4" long.
The engineering numbers match up with the 65 and 66 electrical assembly manuals. Had originally thought that maybe it was the difference between V8 and 6 cylinder ground wires, but that does not appear to be the case based on the assembly manuals. Most likely just an increase in length to address any stretching issues possibly caused by incorrect installation?
Would be interesting to figure out when the transition was from FDD to FDA wires. Also, just FYI... the reproductions only are made with FDA and the longer length as far as I know. There is also a reproduction marked with 'C5ZB' I think, which I don't believe there was ever a marking like that on an original ground wire.