Re: American domestic car market.
First group are Ford Brands, Second one is GM brands. You forgot Chrysler Brands i.e Chrysler, Jeep, Dodge
As Fiasal said that the base is same, even if sometime car looks different, they are built off the same platform. Like PT cruiser and Neon are same Platform (Chassis, Skeleton etc)
The reason is Marketing. Other than the fact that they appear as two different cars to normal consumers, manufacturers can sell the same cars through different dealerships.
When Neon was first released, it had 2 versions of the otherwise identical vehicle, DODGE NEON and PLYMOUTH NEON. Reson was so Chrysler can sell the same vehicle through Dodge dealerships as well as Plymouth dealerships