Paralle Degree in Creating MVW

When using below statement to create a materialized view, I find that it not only creates the materialized view with a DEFAULT degree, but also uses parallelism of DEFAULT degree to create the materialized view.

CREATE MATERIALIZED VIEW DSS_DIM_MVW
TABLESPACE PDW_DATA
NOCACHE
NOLOGGING
NOCOMPRESS
PARALLEL ( DEGREE DEFAULT INSTANCES 1 )
BUILD IMMEDIATE
REFRESH COMPLETE ON DEMAND
WITH PRIMARY KEY
AS
SELECT "......query from multiple tables of DOP 1"
;

On the server with 4 LCPU, the database uses 2 slave sets of each 8 (4 * 2, default) parallel processes to run the statement. So totally, 16 slaves are recuited to run the creation. The two slaves in the first slave set could be scanning a table and passing the results to the slaves in slave set 2 for sorting.

After the query finishes, the materialized view gets DEFAULT as its DOP.