I'm kind of interested as well to know where they get all those extra people in there. Maybe they count extra people in skyboxes or on the sidelines?
I think the extra numbers come for people who work the game. The press stadium workers etc. Also i know that the stadium doesnt know how many people are auctaly at the games. So when they announce the number of people they kind of pull it out of the air.
The extra numbers come from all the non-ticket purchasing people at the game. This includes press, concessions, security, red cross, boy scouts, recruits, cheer leaders, and even coaches and players for both teams. The announced number is the number of tickets sold, plus the number of non purchasing people at the game. The number of tickets sold remains constant because all tickets are pre-sold, it is the extra workers, players, recruits, media, etc... that make the number fluctuate. Because of this the numbers are not necessarily exact, but they are very close (i would guess 20-30 people). We keep the sell out streak going because because even if people don't use their tickets they have been sold so it counts. Also if we don't sell out a game for some reason (rarely happens, last I knew of was 2007) the extra tickets can be purchased at the box office day of and if they are not purchased it still counts as a sellout because of the extra people which push the number to "capacity".
In the north expansion they added 6500 seats while the press box/sky box addition (1999 and added more clubseats in 2000) only added about 1200 seats.