Oracle Database Discussions

Hi everyone.

Why, if I do this in SQLPlus console, this happens?

1. SELECT username, timestamp, logoff_time from dba_audit_trail
This returns, for example: PETE 02/01/2009 03/01/2009

2. SELECT userid, timestamp#, logoff$time from sys.aud$
This returns: PETE *[NULL]* 03/01/2009

So I mean, dba_audit_trail records the loggON date, but their copy in the sys.aud$ NEVER SAVE the logoon date, anybody know why this happens?
Whats the real difference betwen dba_audit tables (trail) and Sys.Aud$.

I need this solution, please.

Thanks in advance!