With $4.6 billion in grosses now projected, the 2013 summer domestic box office will surely set a record as the biggest ever.

So why aren't we hearing champagne corks popping in Hollywood?

After all, the season is on track to finish at least 4 percent ahead of the previous mark of $4.4 billion set in 2011, according to Box Office Mojo, and well ahead of last summer's $4.29 billion.

And there were plenty of hits, led by franchise fare like Disney's "Iron Man 3" ($408 million), Universal's "Despicable Me 2" ($346 million), Warner Bros.' "Man of Steel" ($290 million), Disney's "Monsters University" ($261 million) and "Fast & Furious 6" ($238 million) from Universal.

Also read: Brad Pitt, Horror, Ryan Reynolds and China: The Summer's Biggest Winners and Losers

But not all of the franchise movies clicked. "The Wolverine" ($121 million) and "The Hangover Part III" ($112 million) will be the lowest-grossing entries so far, and "The Smurfs 2" ($58 million) will finish with around half of the original's haul.

Here's an example of just how mixed a bag this season was: Disney and Universal, each of which had two films in the top five, also had two of the season's biggest bombs in "The Lone Ranger" and R.I.P.D."

Also read: The Summer's 6 Biggest Payoffs – and 5 Biggest Bombs

So while it may be the best summer ever at the box office, here are five reasons the studios aren't clicking their heels.

1. Over-the-top overhead: It took 19 movies with production budgets north of $100 million to achieve the mark; last year there were 13. Moviegoers loved the variety of films and that had a lot to do with the record. But the multiplex mash-up brought an unprecedented — and all but inevitable — string of mega-budget flops, including "After Earth," "White House Down," "The Lone Ranger" and R.I.P.D."

2. Original sins: The reason we saw so many pricey movies is that the studios are searching for the Holy Grail of film franchises. All it takes is one to connect — like "Marvel's The Avengers" last summer — to assure billions of dollars in box office, licensing and even theme park revenues for years. But the summer's biggest bombs were all originals, and so far only one sequel — to Summit's surprise hit "Now You See Me" — has been given a green light.

See video: James Cameron, Dan Loeb: The 5 Worst Decisions of Summer

3. Overseas is stagnant: The summer record is based on domestic grosses, but in today's box office world, international returns are critical. The top 10 films earned $4.85 billion abroad last summer, which is a number that summer 2013 will not be able to match (currently at $3.8 billion, it will likely wind up around $4.2 billion). The highest-grossing films at the foreign box office this summer are: "Iron Man 3" ($805 million); "Fast & Furious 6" ($547 million); "Despicable Me 2" ($435 million); "Man of Steel" ($360 million); "Monsters U" ($398 million) and "World War Z" ($319 million).

4. Family film funk: Universal's "Despicable Me 2" is still going strong in its eighth week and Disney and Pixar's "Monsters U" is up to $660 million worldwide. But as with the tentpole movies, there were too many family movies too close to each other. By the end of summer, parents and their pocketbooks were exhausted, and animated films "Epic," "Turbo" and "Planes" all underperformed.

5. 3D malaise: The dew is plainly off the 3D rose, at least domestically, and that's bad news for the studios, which have come to count on the $2-$3 surcharges the format brings. The 3D showings of "Turbo" accounted for just 25 percent of its total box office, which represents the format's worst showing yet, and 3D contributed 30 percent of the $53 million opening of "The Wolverine," a new low point for action releases. ‘Monsters University" (31 percent) and "World War Z" (34 percent) similarly sputtered. It's still working overseas, but you have to wonder how long that will last as foreign audiences become more sophisticated.

On the bright side, the strong summer has drawn the year's box office even with last year's. And with a healthy fall and holiday season, 2013 could top last year's record $10.8 billion haul.