The answer, at least for me, lies in the way the film was marketed. Harrison Ford was not the only one pushing for Deckard to be more human, and to be a hero of sorts, despite the depressing scenery. So did the initial test audiences, who reacted strongly, not only to the confusing storyline (IMHO, I'm putting that one down to low intelligence in the audience, who were probably expecting a "wham-bang let's kill some fucking androids" movie), but to the whole idea that the hero they'd been cheering for all along was really one of the "enemy". A replicant.

They were treated to what is now known as the Director's Cut of the film. After the intensely poor reactions of the audience (whom I'd still like to hunt down with something sharp and deadly), the studio forced Ridley Scott to cut out the scenes that would imply Deckard's mechanical nature, and tack on a "happy ending". Also, to alleviate the problems of slow audiences not keeping up with the plot, explanatory voice overs were added (by an extremely non-enthusiastic Ford, who - in a flash of common sense - deemed this particular addition idiotic).

One of the principal scenes cut was the (in)famous unicorn scene. At some point during the movie, Deckard dreams of a unicorn. At the end, Gaff leaves an origami unicorn in front of Deckard's apartment, just as he is about to run away with Rachael. (Deckard never told Gaff about his dreams, and yet he knows anyway - because Gaff knew from the start about Deck's true identity.)

The unicorn, to me, is what gives it all away. This is also why the Theatrical Version and the Director's Cut are a gnats wing from being two completely different movies. The Theatrical feature is the story of a simple good-versus-evil battle, where the hero wins, gets the girl, and all ends well. The Director's Cut movie, however, stays more true to the original premise of raindrenched depression and the question of identity - only to find out, at the end, that our hero is not what he seems to be at all, and that he'll be dead in less than four years.

Interestingly enough, the computer game based on the movie deals with the issue in a rather novel way. Upon starting the game, it randomly chooses whether or not the protagonist will turn out to be a replicant (as well as several other important characters in the game). Thusly, to see all the endings, you'd have to start over completely from scratch at least once.