You tested a segment and the count seems off. Why?
1. Only contacts in 'normal' status can belong to a segment. Once an email has unsubscribed or gone into 'held' status, it no longer appears in the segment. If you were to export the database, you would still see the criteria for the segment in the appropriate Demographic field.
2. If you noticed at mail sending that the segment count is different, it could be that cross posting has removed duplicate emails if the mailing was sent to more than one segment and the same emails appeared in both segments.