A contact who is marked as Opt Out is still being sent monthly newsletter. They opted out of all emails 3+ months ago but continue to be on sent emails. They have also unsubscribed from the group.
If you are reusing an old email or even just reusing last months which was a copy of the previous month and so on, check to make sure your include list does not have any previous mailings included.
As oppose to "Members of GroupName" it will be listed as "Recipients of OldMailingName"
Recipients of prior emails don't go through the same Opt Out check as a mailing group would. This feature should not be used to major big or regular mailings.