Desire to know more about the container_data attribute of a common user

The "set container_data" clause of alter user can be used to control which pdb's information being visible to a common user.
Be interested in it,I do some tests to verify its function. My tests is composed of two independent tests and I have some questions about my tests. My question is in bold and big fonts.

---My testing environment has 2 active pdbs: orapdba & orapdbb
SQL> select * from v$pdbs;

    CON_ID       DBID    CON_UID GUID                             NAME                           OPEN_MODE  RES
---------- ---------- ---------- -------------------------------- ------------------------------ ---------- ---
OPEN_TIME                                                                   CREATE_SCN TOTAL_SIZE
--------------------------------------------------------------------------- ---------- ----------
         2 4071385469 4071385469 E8B486756A607BCAE0436438A8C029D2 PDB$SEED                       READ ONLY  NO