Some rows of AUD$ can not be deleted by another user than SYS (10.2.0.4)

Hello,

I granted 'select delete' on SYS.AUD$ to system, to be able to delete some of the rows. It works well except that the "DELETE sys.aud$ WHERE ..." commands themselves are stored in the table (action# 7) and can NOT be deleted if I don't use SYS account:

(under SYSTEM account:)
SELECT COUNT(*) FROM sys.aud$ WHERE ntimestamp# BETWEEN sysdate - 3 AND sysdate - 2 AND action# = 7 AND obj$name = 'AUD$';
  COUNT(*)
----------
       130
DELETE sys.aud$ WHERE ntimestamp# BETWEEN sysdate - 3 AND sysdate - 2 AND action# = 7 AND obj$name = 'AUD$';
0 rows deleted.

other example:

SELECT COUNT(*) FROM sys.aud$ WHERE ntimestamp# BETWEEN sysdate - 5 AND sysdate - 2;