I think the statistics go wrong due to the log parsing method.
Lets look how mail traverses inside zimbra:
Inbound internet mail:
Mail arrives -> log
AV, Antispam queues -> log
Local delivery -> log
In/outbound internal mail
Local av loop -> log
Local delivery -> log
Outbound internet mail
Local av loop -> log
smtp delivery -> log

Stars mark log points
You can see the mail multiplying in statistics by factor of 2 or 3 per every mail, the count could be even more, maybe 3 for internal mail, 4 for internet.
So the totals count in statistics is wrong by factor of 2-4 and it's not easy to correct.
It would require rewrite on the report gathering side to fix it.
But it's not all that simple!
Another thing mixing statistics is nature of SMTP itself.
You can send email, but receiving server can request delay and retransmit(think postgrey).
This would log same message being sent again after few minutes, maybe even several times during same day before it was actually delivered.