Consider the following scenario.
You create a meeting that includes a distribution list (DL) as an attendee.
You later open the meeting and notice that an attendee is listed separately, even though they are also a member of the DL.
You remove this individual attendee.
When you close the meeting, you are prompted to save changes and send an update. When you do this, the individual attendee who was removed from the attendee list receives both a meeting cancellation and a meeting update.
If the attendee has a Microsoft Exchange Server 2007 or Exchange Server 2010 mailbox, the Exchange Calendar Attendant processes the meeting cancellation and update automatically. Because the meeting cancellation is received before the meeting update, Outlook marks the cancellation as Not Current or out of date. As an out-of-date item, it is automatically deleted. When the most recent update is processed, the meeting is put in the tentative state in the recipient's calendar. This occurs even if the recipient previously accepted the meeting.
The behavior that is described in the "Summary" section is by design. A meeting organizer may notice that an individual attendee is also a member of a DL. In this case, the organizer may try to clean up the attendee list by removing the individual attendee. If the organizer removes the individual attendee and then sends the meeting update, the attendee receives two meeting updates. The first update is a cancellation. The individual attendee received this because they were removed from the meeting. The second update is a meeting update that the attendee receives as a member of the DL. This makes sure that the meeting remains in the attendee's calendar, as expected.
When an attendee is removed from a meeting that contains a DL, Outlook does not display the prompt Send updates only to added or deleted attendees. If Outlook let this occur, the individual attendee would receive only the meeting cancellation. This is not desired behavior and is why Outlook displays only the Save changes and send update prompt.
By default, when you create a new meeting, the Request Responses option is enabled. When an attendee accepts or declines your meeting invitation, a meeting response is automatically sent to you. Meeting responses are processed by the Exchange Server or Outlook client. If the attendee is a member of a DL, they are added to the attendee list as an individual attendee. This lets their response be tracked separately.