Why is Schema for Database Diagram not dbo?

SQL Server Administration
1

I have a login named test which has the db_owner role. If that login creates a database diagram named Diagram1, the name of the diagram is test.Diagram1.
But if test creates a table name Table1, the name of the table will be dbo.Table1, it inherits the dbo schema.

Why are database diagrams getting a different schema owner? This is MSSQL 2014

Thanks.

2

for your user id in the database, what is defined as your default schema?

3

Thanks for your reply. The default schema for the user is dbo.