On my ADW instance, indexes created by auto indexing are compressed by default but manually created indexes are not.
Why are manually created indexes not compressed by default?
Test case:
==========
SQL> create table a (a number);
Table created.
SQL> create index a_ind on a (a);
Index created.
SQL> select INDEX_NAME,COMPRESSION from dba_indexes where index_name='A_IND';
INDEX_NAME
--------------------------------------------------------------------------------
COMPRESSION
-------------
A_IND
DISABLED
SQL> drop index a_ind ;
Index dropped.
SQL> create index a_ind on a (a) compress;
Index created.
SQL> select INDEX_NAME,COMPRESSION from dba_indexes where index_name='A_IND';
INDEX_NAME
--------------------------------------------------------------------------------
COMPRESSION
-------------
A_IND
ENABLED