db_owner default selected when create new login in SQL 2008 R2

Hi Friends

I face the below issue on SQL 2008 R2.

When create new login, db_owner membership selected by default for 1 database. If untick and click ok, system prompts the error message, but added the login without ‘db_owner’ membership.

If not untick and click ‘ok’, system add the login but not prompt any error. After check the Properties of the new login, the ‘db_owner’ member unticked (By System).

Server Role is ‘Public’.

Any one tell me, why the ‘Database Role Membership’ - ‘db_owner’ selected default for the DB upon create new login?

Thanks in Advance