The Number Two Flop

Why is it, in this day in age, that a movie that doesn't come in number one in box office totals on it's very first weekend is considered a flop? For instance, I was just reading about a movie (Forgetting Sarah Marshall) that I want to see in the theatre. It did not come in at number one and the tag of the article I was reading said "Marshall gets dumped." It only made 17 MILLION dollars and opened in about 2900 theatres, that's 250 less than the number one movie, The Forbidden Kingdom. That movie made about 20 MILLION dollars. Let's average that out. $6623 for Kingdom, $6200 for Marshall per theatre, using the exact numbers from Yahoo!. A difference of about $400, which is roughly 40 people per theatre during the entire duration of the weekend, which is Friday through Sunday. Now let's say that there's ten showings per day and that's less than one more person at each showing. Broken down like that it doesn't seem like there is a big difference, if one more person had been at each showing of Marshall then it would have won. The point is, from what I gather, about 89% of the people who saw Forgetting Sarah Marshall this weekend, that's about 1.5 million people, liked the movie, so just because it debuted at number two doesn't make it a flop.