Oracle Audit: Why Logons are less than 0.1% of logoffs?

I see the following quantities of logins versus logoffs vs logoffs by cleanup in a database:

SQL> select action_name, count(*) qty
from Dba_audit_session
group by action_name
order by 1;  2    3    4

ACTION_NAME                        QTY
--------------------------- ----------
LOGOFF                         1946180
LOGOFF BY CLEANUP               754683
LOGON                             1026

It doesn’t make sense that logons are less than 0.1% of logoffs.

Any ideas why?