Hércules de Alicante Club de Fútbol, S.A.D. (] ) is a Spanish football team based in Alicante, in the autonomous community of Valencian Community. Founded in 1922, it currently plays in Segunda División B – Group 3 and plays its home games at the 30,000-capacity Estadio José Rico Pérez.
Estadio José Rico Pérez can seat 20,000 spectators without filling up. 
The context says that Estadio José Rico Pérez has a capacity of 30,000. That means that if 20,000 spectators were present, the stadium still would not be full. The system likely thought that the statement was incorrect because the number 20,000 does not appear anywhere in the context.