When monitoring a tab with dlg.watchtab for add, change, delete events, the addition of an item triggers an add event (Msg.data = 1) followed by change event (Msg.data = 4) a couple of seconds later. Deleting an item behaves as expected, triggering a single delete event (Msg.data = 2).
The manual reads..
"For the filechange event, the Msg.data property contains a bit mask indicating which file events occurred. 1 = add, 2 = delete, 4 = change. The values will be added together (so e.g. 6 indicates at least one item was changed and at least one was deleted)."
..which means that this is seen as two discrete events.