Paul,
It seems that you had just tried to supply the wrong number of columns in the original insert, and has nothing to do with newsequentialid() (it could have been any datatype with a default). If you don't specify the list of columns to be inserted, then SQL Server will expect all of the non-identity columns to be supplied by the subsequent value-providing clause.
I'm surprised you didn't get this error:
Column name or number of supplied values does not match table definition.
Specifying the column list (always a good practice) told SQL Server which column(s) to expect and to use the default value for any other column(s).