Normally when an organization sells tickets they are looking for revenue in my eyes. I would assume there are many chairbacks that are sold as season tickets that are left empty as they may have been sold to companies (insurance companies as example) and have no vested interest in attending. The CEO aint going to the game and the said company puts out an email to employees - who wants tickets to this weekends UConn football game - no one in the company wants them as the product is bad, the email only goes out to executives, upper management, whatever. Those tickets are sold but people arent using the tickets so the seat is left empty. The revenue is in hand.
If you are looking for people to actually use the seat, thats another battle all in itself.
I guess the point is there may be times when 30,000 tickets are sold but the attendance is 25000. Ever ticket sold doesnt mean every seat filled.