Attendances given out are the number of tickets sold. The police do their own checks to plan for policing for future games. If season ticket holders don’t turn up they are still included in the attendance figure.How would they know? Aren’t attendances based on tickets scanned at turnstiles?
Article from a while back - https://www.followfollow.com/forum/threads/old-trafford-empty-seat-stats.266811/