Continuity mistake: When Chester is trying to get Matthew to come into his car after school, Matthew has a football in his hand for a couple of shots. Then, the football disappears and his replaced by his backpack for the last shots of the scene.
Continuity mistake: Towards the end of the film when Penny Pester says, "This is for you, Coach Chester" and gets up enough courage to take the soccer ball down field, the camera pans to her legs kicking the ball and pulling off all kinds of spectacular offensive moves. The legs of the person, however, are much thinner and less tan than Penny's legs - obviously a stunt double.
Answer: Maybe they just thought he had a deep voice for a girl. I also thought about that, like when the girls hug Martha he's a guy - no boobs - but it's just a fun movie.