Here's an apropos article:
Box office is not poker. It’s not about new releases and holdovers attempting to make more money than the other movies in the marketplace. Box office is blackjack, where it’s only the film itself against ‘the house’. ‘The house’ in this case being an individual film’s budget and specific (reasonable) expectations about what would qualify as success. If Pacific Rim opens with $30 million, it will arguably not be a domestic success even if it’s number one for the weekend. If Pacific Rim muscles its way to $50 million, it will arguably be a success whether it’s in first place of tenth place. The individual numbers are what counts. It doesn’t matter how high a film debuts, nor does it matter how long it stays in the top five or top ten of the box office. It’s about how much money a specific film makes in relation to how well that specific film has to perform.
And come Sunday morning, Pacific Rim and Grown Ups 2 should be judged not over which film topped the other, but by how much each film debuted with in relation to their own specific box office needs. The studios don’t care how much all of the films in the marketplace made in a given weekend if their film tanked. And studios don’t care if their film was number one or number two, as long as the actual weekend gross is to their liking. Paramount was thrilled that World War Z opened with $66 million last month and they surely didn’t care too much that Monsters University was still the top film of that weekend. And if Pacific Rim can open substantially higher than its tracking (and/or make a killing overseas), Warner Bros. surely won’t care too much if it makes more or less than the likes of Grown Ups 2 or Despicable Me 2. And we shouldn’t care either.
Reporters turn things like political races and movie box office into horse races because they believe it'll boost their own ratings if they can make the news look like sports. So they manufacture head-to-head competitions where none exist.