The DNC knew that some of the 5000 delegates and 15,000 media people would be out scouting free food and partys, so they hired shills to fill the empty seats. Apparently the shills knew how to work the system better.
With an event that has definite ticketed attendees, there is no way the venue could be overcrowded unless they played games with the tickets.
There are 5000 delegates and 15,000 from the media. Therefore no room for the union thugs and college kids bused into Charlotte.