...yup. This pretty much explains it;
"The Mets announced that the attendance at their game Thursday night against the Phillies was 20,010. Even a casual glance at the game on television, though, suggested that there were no more than a few thousand fans at Citi Field. Were the Mets fibbing?
Technically, no, because teams in Major League Baseball and other sports leagues report the number of tickets sold, not the number of tickets used. The bulk of the unused tickets were held by fans with season-ticket plans, including partial season-ticket holders who as part of their package had to accept tickets to midweek games held at the beginning and end of the season when school is in session and the weather is chillier.
Teams know how many fans attend their games on a given night, a process made easier by bar codes scanned at the turnstiles. But teams, the Mets included, rarely disclose those figures, which they consider proprietary.
Estimating the number of fans attending a game is not easy. At any given time, some fans are shopping for merchandise, taking a bathroom break or waiting in line for food at concession stands. Because the concourse on the field level at Citi Field is accessible to everyone, some fans buy tickets for seats in the upper deck but never go there, and instead hang around the field level. Some fans in suites, clubs and restaurants are out of view.
Until 1999, National League clubs reported attendance based on turnstile counts and the American League teams reported paid attendance. In 2000, all clubs started reporting the number of tickets sold because those figures were used to calculate revenue sharing between the clubs, according to Major League Baseball."