Because like Wikipedia, anyone can edit IMDB. Not to the same extent as Wiki of course but you can still add stuff to the database(Movie quotes, goofs, trivia, etc.). One example I can think of as to why IMDB isn't all that reliable is that according to them, a Dragonball Evolution sequel is scheduled to come out in 2011. Now there's more speaking against this rumor than there is speaking for it: the movie tanked at the box office, it garnered mostly negative reception, lost Fox a whole lot of money, and is universally hated by fans of the original Dragon Ball franchise, myself included. Do you really think a company would do a sequel to a movie that performed horribly at the box office and risk losing even more money?
I'd like a Land Before Time 14 just as badly as anyone else here but right now, it looks as though we won't be getting a 14th movie. I knew the series would end eventually...I just wish it hadn't ended like this.