Everyone loves movies that are based on a true story, because it means you can say things like: "What a great movie... and it's based on a true story!" For some reason, we human beings like it when we know that things have happened in real life at some point - if said "true story" ended in a happy way, it helps us to inject a little hope into our lives; if it ended in a bad way, well... we've all got a morbid fascination with the sick and perverse, too, right?
Hollywood has made hundreds of movies "based on true stories," of course, many of which are fairly accurate in their depiction of actual events - enough to justify having a "based on a true story" tag at the beginning, anyway. Take 12 Years A Slave, for example, which - for all intents and purposes - was an admirable attempt to convey the events outlined in Solomon Northup's diary with little artistic or creative embellishment made. It's about as "true" as we can probably expect.
Sometimes, though, certain movies being sold as "true stories" aren't true at all. In fact, they're the complete opposite, and have presumably been branded as such in the hope that people will come out of the theatre saying things like - you guessed it - "What a great movie... and it's based on a true story!" Which, in turn, is supposed to inspire other people to go and check it out, 'cause aren't true stories amazing?
What's really amazing is how much filmmakers get away with this...