[442w30]

To your comparison, you're using extremes.

They're both A-bodies.

But, instead, compare a Cutlass with a LeMans. Same car, different styling.

But let's use a Mopar for a metaphor because using GM introduces complications due to engine and, hence, identity.

A Coronet and a Belvedere are the same car. They are of the same design but have different styling. Aside of a stylist's pen, they are the same car with the same design constraints headed by the same chief of design.