Susan, it's about history. Some curds were drained or drained and pressed, and had a bottom. The cheese was left and pressed to form. Some were just hooped to form the shape and the top was pressed with a follower. French lactic cheeses that are flipped repeatedly, for example, do not have bottoms. And ones that are left to drain do. Molds that are pressed tend to have fewer holes on the bottom that for cheeses that are not pressed.
It's actually about utility and history. And yes, generally, they are fairly interchangeable.