Factory BBP on 7.25 axle

My '73 318 disc brake Dart Sport is BBP with a 7-1/4.

The '74 318 Valiant with disc brakes is BBP is an 8-1/4

The '75 318 Duster with disc brakes I had previous was BBP with 8-1/4.

I don't know what the criteria for axle choice was exactly.