CPU time in v$sesstat

Could someone tell me why CPU time recorded by oracle (statistic# 12) is 0 for all background processes, while CPU time for the same processes as recorded by O/S is not 0?

select spid, value, c.program
 from v$sesstat a, v$process b, v$session c  where paddr = addr
 and a.sid = c.sid
 and statistic# =12
 and type = 'BACKGROUND';

SPID VALUE PROGRAM

--------- ---------- ------------------------------------------------
417                0 oracle_at_sugar.ma.intuit.com (PMON)
419                0 oracle_at_sugar.ma.intuit.com (DBW0)
421                0 oracle_at_sugar.ma.intuit.com (LGWR)
423                0 oracle_at_sugar.ma.intuit.com (CKPT)
425                0 oracle_at_sugar.ma.intuit.com (SMON)
427                0 oracle_at_sugar.ma.intuit.com (RECO)

6 rows selected.

ps -eo pid,time,comm | grep _$ORACLE_SID |sort

  417        0:00 ora_pmon_srdb1
  419        0:00 ora_dbw0_srdb1
  421        0:02 ora_lgwr_srdb1
  423        0:17 ora_ckpt_srdb1
  425        0:01 ora_smon_srdb1
  427        0:00 ora_reco_srdb1