Archiving fga_log$: sys.dbms_audit_mgmt.clean_audit_trail doesn't delete anything
As Mat has pointed out, I hit Bug 9164488 (DBMS_AUDIT_MGMT.CLEAN_AUDIT_TRAIL does not delete sys.aud$ and sys.fga_log$ tables [ID 9164488.8]) which should be fixed in Release 11.2.0.2.

In the meantime, running on 11.2.0.1, I was able to circumvent the bug with a purge job in order to do the work. As soon as I ran 

begin
  sys.dbms_audit_mgmt.create_purge_job (
     audit_trail_type           =&gt; sys.dbms_audit_mgmt.audit_trail_fga_std,
     audit_trail_purge_interval =&gt; 999,
     audit_trail_purge_name     =&gt;'Purge_Test',
     use_last_arch_timestamp    =&gt; true
  );
end;
/


clean_audit_trail would do the work, as previously expected.