why WRH$_ACTIVE_SESSION_HISTORY only have one partition

Hi,
why WRH$_ACTIVE_SESSION_HISTORY only have one partition? while the others have plenty for below sql.

set serveroutput on
declare
CURSOR cur_part IS
SELECT partition_name from dba_tab_partitions
WHERE table_name = 'WRH$_ACTIVE_SESSION_HISTORY';

query1 varchar2(200);
query2 varchar2(200);

TYPE partrec IS RECORD (snapid number, dbid number);
TYPE partlist IS TABLE OF partrec;

Outlist partlist;
begin
dbms_output.put_line('PARTITION NAME SNAP_ID DBID');
dbms_output.put_line('--------------------------- ------- ----------');

for part in cur_part loop
query1 := 'select min(snap_id), dbid from sys.WRH$_ACTIVE_SESSION_HISTORY partition ('||part.partition_name||') group by dbid';