I used a General Mailbox Traffic report template (Templates > Mailbox Traffic menu) and selected SERVER1 for December, 2016 last month. The report I obtained displayed 51,000 received messages.
Today, I run the same report for the same period and I obtain 54,000 received messages.
Selecting mailboxes by server does not mean that you will obtain the mailbox traffic on SERVER1 for December, 2016: you will actually get the traffic of mailboxes located on SERVER1 at the time when the report was run.
It is more than likely that SERVER1 now hosts a few more mailboxes than last month, and that these mailboxes received 3,000 messages in December, 2016.
If you wish to obtain a report on SERVER1's traffic, you should run a report from the Templates > Server Traffic group.
The difference between both types of reports is that the General Mailbox Traffic report template deals with the traffic of mailboxes selected by a particular attribute (server), whereas templates of the Server Traffic menu deal with the actual traffic of the selected server; it is based on traffic events imported from that particular server, and therefore it will not vary over time.