You are right.

The single user login co-exists with the windows group login.

It looks that makes the difference.

They co-exist is because sometimes that specific user does need some more priviliges than those in the windows group.

I assume the default schema for both the user and the windows group should be dbo.But apparently not.

If they co-exist, is it a required step we have to go to the single user account and set it to dbo?

I early tried to set windows group default schema following this for we are using SQL 2012, it does not work.

http://sirsql.net/blog/2012/3/12/sql2012-its-the-small-thingsdefault-schemas-for-groups.html