I think it's because you have up to four different trim levels on essentially the same vehicle being called four different models (Biscayne, Bel Air, Impala, and Caprice). In the Chevelle line, you could have the 300, the 300 Deluxe, the Malibu, and the Concours (not to mention another name or two for the wagons), but they are all Chevelles. Another term referring to the various full-size cars is merely 'Passenger'. Clear as mud, right?
