Statistics - why is the total number of calls not adding up with Answered and Hung up in queue?

In the queue report, Total Calls are the total number of calls that have been routed through each queue.
One incoming call is counted in all queues it has been routed through. Total calls may be greater than the sum of Answered, Hang up, Timed out, Exiting and Callback, because there are other queue exits, e.g Full and No agents logged on. By clicking the customize button on the report you can add "Empty (No agents logged on)" and "Max in queue" to your report.