Filmed over 20 days? Andrew Keegan ages noticeably.

The trivia section mentions that the movie was filmed over the span of 20 days, yet Andrew Keegan's age seems to jump around quite noticeably. It's most noticeable from the party (so young he's barely recognizable) to the first time they're shown playing softball together (looks almost as old as he did in 10 Things I Hate About You). He seems to have aged 2-3 years during filming. I'm wondering if they did some reshoots and that would account for his noticeable aging?