“…I think that read consistency is not ensured for queries on data dictionary and dynamic views.. …”

Chapter 6 “Data Dictionary and Dynamic Performance Views” of Oracle® Database Concepts 11g Release 2 (11.2),
see
http://docs.oracle.com/cd/E11882_01/server.112/e25789/datadict.htm
says
“… Dynamic performance views are based on virtual tables built from database memory structures. Thus, they are not conventional tables stored in the database. Read consistency is not guaranteed for the views because the data is updated dynamically. ”

Hence, I expect the data dictionary to respect read consistency.

The difference does not depend on the execution plan:
SQL> select
2 (select count(*) from user_tables) cot,
3 (select count(*) from user_objects where object_type=’TABLE’) coo
4 from dual;

COT COO
———- ———-
0 0

SQL> create table t
2 as
3 select
4 (select count(*) from user_tables) cot,
5 (select count(*) from user_objects where object_type=’TABLE’) coo
6 from dual;

Table created.

SQL> select * from t;

COT COO
———- ———-
0 1