I believe PM's are gathered from the members outboxes by the server on a timed basis. I don't know what the time period is, but lets just use an hour for an example on the hour. If you send a PM at 10:05 AM and some else sends one at 10:55 AM, both will go out at 11:00 AM. One took 55 minutes to go out and the other took 5 minutes.