[Katshot]

Cars with 6-bolt wheels were built on the V4U/B9Q "commercial" chassis, and the ones with 5-bolt wheels were built on the W20 "livery" chassis. There is a world of difference between the two but the main reasons for using the lesser W20 chassis were:
1. Cost (the W20 chassis is MUCH cheaper and doesn't force the customer to pay the dreaded "gas-guzzler" tax.
2. Ride quality (the W20 car is designed to be driven "as is", whereas the others are designed to have a lot of weight added to them, hence the extra heavy-duty components)
3. The cost of repairs on the commercial chassis cars is much higher and appearance upgrades such as wheels are virtually non-existent.