When you send a meeting request, you can put Distribution Lists (DLs) on the To line as required attendees. When the recipient accepts this meeting invitation, your original meeting item shows that recipient as an Optional attendee instead of Required.
When Outlook receives a meeting acceptance, the sender of the acceptance is compared to the recipients in the meeting attendee list. If that recipient is not listed, it is assumed to be "optional" and added to the list of attendees.
This is by design.
To work around this issue in Outlook 2003 or later versions, expand the DL as soon as it is added to the meeting list. If the DL contains other DLs nested in it those will also have to be expanded.