There are often seats kept for various reasons (team families, opposing team, dignitaries, set aside just in case, that are released late. When I go to a show, I always wait until the day of and in most cases (not that I go to shows often) there are front row seats available that were not claimed by family and friends.
With the Vision Campaign, I'd imagine that they held back a number of tickets for big donors or people they are/were wooing...That is how any business works.