Roundworms are among several phyla of animals known as pseudocoelomates. They are so named because they have a pseudocoelom, a hollow, fluid-filled cavity that is lined by mesoderm on the outside and endoderm on the inside.
I am thinking that they just differ in their physical structures; both contain pseudocoeloms
The coelom is the body cavity of an animal. A true coelom is a body cavity that is complete from mouth to anus with no breaks; there are animals that have pseudocoeloms - mouths and anuses but no solid connection between the two.