If you go back two years back, no one quite knew if Inception is going to do big business. They knew it was going to probably make its money back, based on Nolan's reputation. But, other than that, it was a wild card. Many analysts called it a "difficult sell" (some even predicted a flop). But the relentless campaing paid off.
As I say, these kind of things rarely take off the way, say, Batman film would. It has no launching platforms. And filmmaker's name is never as strong as you might think. Hardly a selling point for a massive blockbusters.
And Leo? What kind of massive releases did he have? Besides, Titanic, that is?
Another major difference is Inception was never, ever a sleeper which slowly earned its popularity and status due to slowly spreading positive word of mouth in a time before mobile phones and Facebook. Shawshank was never a fashion accessory film, it was never viral.
It wasn't a sleeper, but neither was it a sure hit. Great carte blanche vanity projects from directors rarely make a lot of money. Besides, the film wasn't related to a popular brand, wasn't a sequel, and didn't have a 3D release (or anything like that) The trailers were actually quite vague and nobody quite knew what it was all about. The only thing it had is that the ad campaign resembled slightly The Dark Knight one. So yeah, word of mouth and solid early reviews did wonders.
I thought it was going to break even, but no more than that.