It was supposed to be close.

"Pitch Perfect 2" and "Mad Max: Fury Road" were both expected to open in the mid-40s, with the a cappella musical having a slight edge. In fact, "Mad Max" did open in line with predictions, grossing an estimated $44.4 million. But "PP2" opened with an estimated $70.3 million -- about $15 million more than the most optimistic projections, and about $5 million more than the original 2012 movie earned during its entire North American run.

This sort of thing almost never happens. But perhaps better than asking how "PP2" became such a runaway success might be asking why everyone so grossly underestimated its chances.

Some possible reasons:

It's the women, stupid.

Both the industry and the pundits who watch it have a long history of accepting the conventional wisdom that female-driven movies don't open big, and that female moviegoers don't buy tickets. Every time a movie comes along that proves this notion wrong (from "Bridesmaids" to "Cinderella"), it's considered a fluke, rather than evidence of an underserved audience responding to one of the few well-executed movies tailored to its interests. (As it turned out, the "PP2" audience was 75 percent women and girls.) So the Hollywood studios simply doesn't make many such movies because it assumes they'll fail, and on the rare occasion that they do make one, they're always surprised when it's a hit.

A corollary to the notion that women don't sell (or buy) tickets is the idea that overseas audiences aren't interested, either. But "PP2" had already opened at No. 1 in Australia (perhaps not coincidentally, home of Aussie native and "PP2" star Rebel Wilson), so pundits shouldn't have been shocked that it would do proportionally well here, either. Considering how foreign grosses now drive Hollywood's filmmaking choices, maybe the studios should recognize that the international success of a movie like "PP2" isn't any more of a fluke than it's domestic success.

The off-screen fanbase.

The first "Pitch Perfect" may have been just a modest hit in theaters, but it had a huge life beyond its theatrical run, thanks to word of mouth that has only escalated over the past three years. It sold $100 million worth of DVDs (this at a time when the DVD market is supposedly dead), spawned a smash soundtrack, and was replayed endlessly on premium cable. No doubt these were all factors in greenlighting a sequel, but still, it's not apparent from the first film's ticket sales alone how large and avid a fanbase the Bellas have. It's too big to be dismissed as just a cult.

The execution.

Perhaps pundits saw the other female-driven movies currently playing -- notably, "Hot Pursuit" and "The Age of Adaline" -- and thought the market was already saturated. But "Adaline" isn't a comedy, and audiences didn't much care for "Hot Pursuit." If moviegoers were hungry for a comedy that actually delivers -- no matter whether it's male- or female-oriented -- "PP2" had the market all to itself. The movie earned a very high A- at CinemaScore, suggesting that audience are delighted and that word-of-mouth is strong.

The talent.

Wilson's Fat Amy was the first film's breakout character, and the fearless comic actress ups the ante this time. Co-star Anna Kendrick is relentlessly charming on and off-screen, and she's been tirelessly promoting the film in magazine interviews and talk-show segments that have gone viral. And director/producer/co-star Elizabeth Banks confirms the savvy that marked her production of the first film. A lot of Hollywood was wondering whether she could pull it off and turn her first directing project into a hit -- again, largely because Hollywood is always fascinated when a woman steps behind the camera, since it happens so seldom -- but now, you can bet she'll be asked to helm "Pitch Perfect 3" in a heartbeat.

Indeed, rather than being stunned by the success of "PP2," we should be marveling that "Mad Max" did as well as it did. After all, it's the sequel to a franchise whose last installment came out 30 years ago, its lead isn't a proven box office draw (sorry, Tom Hardy fans, but it's true), it's a hard R that's extreme violence is surely keeping some viewers away ("PP2" is rated a more welcoming PG-13), it's opening against a still-strong "Avengers: Age of Ultron" (which came in third this weekend, with an estimated $38.8 million), and while critics have raved about "Fury Road," audiences haven't been as impressed (it earned a so-so B+ from CinemaScore). For a movie to have all those strikes against it and still open in the mid-40s is a stellar achievement.

One nice paradox: For months, this column has noted that the studios' strategy of counterprogramming almost never works. Just because a testosterone-heavy action film is opening doesn't mean that women will feel compelled to come to the multiplex as well if a female-driven film is opening as well. (Last week's lackluster debut of "Hot Pursuit," opposite the still-massive "Ultron," bore this out.) This weekend's results, however, show that counterprogramming can work if both movies are well-made and compelling enough. But it's the guy-friendly "Mad Max: Fury Road" that was the counterprogramming, while "Pitch Perfect 2" was the main event.