--syslog_name filter of pflogsumm does not work

Issue

When running pflogsumm, it does not show the expected result, there is no message corresponding:

perl /usr/sbin/pflogsumm --syslog_name=outbound_secure  /var/spool/log/mail
Postfix log summaries for Feb 10

Grand Totals
------------
messages

      0   received
      0   delivered
      0   forwarded
      0   deferred
      0   bounced
      0   rejected (0%)
      0   reject warnings
      0   held
      0   discarded (0%)

      0   bytes received
      0   bytes delivered
      0   senders
      0   sending hosts/domains
      0   recipients
      0   recipient hosts/domains

while I should have messages:

# perl /usr/sbin/pflogsumm --syslog_name=outbound /var/spool/log/mail
Postfix log summaries for Feb 9

Grand Totals
------------
messages

     12   received
     19   delivered
      0   forwarded
      4   deferred  (14  deferrals)
      3   bounced
      0   rejected (0%)
      0   reject warnings
      0   held
      0   discarded (0%)

 161232   bytes received
 171662   bytes delivered
      4   senders
      3   sending hosts/domains
      8   recipients
      7   recipient hosts/domains