Very likely based on ‘owners desire’. Back then you could pretty well custode your car with any option. Sometimes the type of rear end was included in an option package if ‘heavy duty cooling’ or ‘towing package’ or ‘heavy duty suspension’. Or if you wanted a certain ratio rear end you had to get a different type of rear end. These days cars can’t be customized nearly as much without a package including dozens of items. And perhaps it was a dealer/fleet/promo order vehicle?