Why does Fenway Park have a sell-out capacity of 38,805 people, but routinely the listed attendance is just?

I think the actual seating capacity (the number used to determine if a game is a sellout) is more like 36-37 thousand. 38,805 is the most people they can legally have inside the park (including people with standing room only tickets and park employees) due to fire regulations. They were able to increase that figure by a thousand or so with some improvements that were made to the sprinkler system over the off season.