The agent or the device might be rejecting the call, by the device I mean that you might have a queue member configured that is not reachable for whatever reason (network problems), if the queue sends the call, and the device rejects the call, it might count as a rejected call if the time for the rejection is more than zero seconds (usually a misconfigured device will reject at zero seconds, those are ignored, but if its 100 milliseconds, then it will count as a rejection. Also, if an agent has a phone set to DND for example, will generate those rejection events (in queue_log is a RINGNOANSWER event). In any case, inspect your /var/log/asterisk/queue_log file and look for those events.