Difference between Owned Schemas and Role Membership for Microsoft SQL Server
Apples and Oranges. Roles are roles and schemas are schemas. The fact that there is a role called db_accessadmin and also a schema called db_accessadmin does not mean a role is a schema nor that a schema is a role. Roles are security membership containers, a principal is member of a role. Schemas contain database schema bound objects and are owned by a principal. When you create a new user you can choose his default schema, add him to certain roles, and grant him ownership of schemas. Although the actions are separate, the designers of this dialog feel they are somehow related as to merit all that UX real estate (that is questionable, but a completely different topic).