It was related to when the car was built and possibly other options, as well.

My old 66 Barracuda (273-4/4sp) had a 3.23 geared 7-1/4 until I sheared the splines off one axle (previous owner had done the same to the other axle AND broke two sets of spider gears). That's when I put the first A-body 8-3/4 (3.23 Sure Grip) I had acquired under it.

The new rear had come out of a 67 GT with the Rallye Package (handling package/disc brakes), 273 four barrel, console, tach, etc.