I see this when deleting an appointment when no reply is sent to the invitation. We're sending a DECLINE notice. I think this may have to do with un-replied to appointments being marked as tentative in the calendar, just as appointments you reply to with a Tentative status are marked. However, I'm not sure. I've submitted this as a feature request.
Derek Curtis
CCO
SmarterTools Inc.
www.smartertools.com