Is dbo the default schema when querying if it is not specified?
According to the Books Online:


  The value of DEFAULT_SCHEMA is ignored if the user is a member of the
  sysadmin fixed server role. All members of the sysadmin fixed server
  role have a default schema of dbo.


So it seems the account used where the default schema is not working is a sysadmin role member. The default schema of the dbo user is always dbo and cannot be changed.

The best practice is generally is to schema-qualify object references to avoid ambiguity and improve performance.