NEW YORK (CNN/Money) - Never mind the forced grins of executives over at Vivendi Universal Entertainment as they talk about how, ahem, thrilled they are with how "The Hulk" did in its opening weekend. Ang Lee's brooding monster film is one of Hollywood's biggest disappointments this summer.

Clocking in at $62.6 million at the domestic box office last weekend, Hulk took in less than "The Matrix Reloaded," "Finding Nemo," "X2: X-Men United" and "Bruce Almighty" did in their debuts this year. Heading into the summer movie season, only "Matrix Reloaded" was expected to do better.

It's not easy being green

What happened? With hindsight people can say the movie was too talky, moviegoers needed a break from action flicks, Hulk looked too much like the Michelin Man, and everyone was too busy tearing into the first few hundred pages of the new Harry Potter book to go to the movies. But even as late as last week, people expected Hulk to be a smash.

Last week on the Hollywood Stock Exchange, where players trade (using play money) domestic box office expectations, "The Hulk" went for as much as $202.45, meaning the market expected it to bring in $202.45 million in its first four weeks. On TradeSports, a Dublin-based betting exchange, investors were giving better-than-even odds that "The Hulk" would gross over $72.5 million in its opening weekend.

Nor is "The Hulk" an outlier here. Figuring out what movies are going to make with any degree of accuracy is an almost impossible job. Back in late April Hollywood Stock Exchange traders thought that both "Hollywood Homicide" and "Dumb and Dumberer" would make north of $50 million in their first four weeks. Both will be lucky if their entire domestic box office take comes to $30 million. They thought that "Nemo" would make around $150 million in its first four weeks. "Nemo" still has a few days before it hits the four-week mark and it's taken in $227.9 million. The little fish that could looks like the summer's top grossing movie.

The art crowd loved it

According to University of Iowa business school professor Tom Gruca, Hollywood Stock Exchange traders tend to be off on their box office bets by about 60 percent. Nor are private forecasters any more reliable.

"There are just so many things that can affect how a movie does," said Gruca. "Everything from the weather to who is in the NBA finals on a Friday night can make a movie go in the tank."

Even though most of the studios are owned by big media conglomerates with multiple revenue streams these days, the unpredictability of movies can give investors big headaches. "Treasure Planet" cost Disney almost $140 million to make and then flopped when it opened last Thanksgiving Day weekend. When Disney said in early December that it had to lower its calendar third-quarter 2002 earnings by 2 cents a share on account of the flop, its shares took a 4.6 percent haircut.

The surprises can go the other way, obviously. Witness the 5.9 percent move higher in Pixar shares since the animation studio's "Finding Nemo" was released May 30.

Just swim to profits

"Some people on the Street were very pessimistic about 'Nemo,' and that drove the stock down," said Gerard Klauer Mattison analyst Jeffrey Logsdon. "Then the movie opened strong and has had healthy weekly results."

Unfortunately, thinks Iowa's Gruca, forecasting movies isn't going to get any easier. Part of the problem is that studios are always trying to market movies in new ways, which means you can't reliably use rules of thumb to make a prediction.

The latest hitch in the game with big movies is to push them onto as many screens as possible on their debut, which inflates their opening weekend take, but also means they tend to see a big drop-off the following weekend as the next hoped-for blockbuster gets rolled out.

"Everybody is competing for the same core audience," he said. "That's making it harder and harder to do the forecasting."

Next weekend Hulk will be vying against "Charlie's Angels: Full Throttle," a movie with huge crossover appeal (grrrrl power plus Cameron, Lucy, Drew and Demi). The green guy doesn't stand a chance.