Historical films are a staple of Hollywood, often drawing huge budgets and, more importantly, huge profits. Some films stay true to their time period and their characters, with films such as Lincoln or Agora providing exceptionally interesting stories with few real changes to the history.
Others, however, have no quandaries about changing dates, numbers, characters, and occasionally, history altogether. Whether it's confusing one person's story with that of another, or mixing up entire civilisations, Hollywood likes to play it fast and loose with what really happened.
Most of the time, directors will push these changes under the bracket of "artistic license," or try to argue that they're attempting to tell the story, not the facts. This is often because a director wants to spin a movie a particular way, ignore what really happened, or because they don't think it would make a good movie.
The problem with this is that more often than not, the facts turn out to be a far more interesting story than what audiences end up receiving. This usually results in a mix of authentic moments blended with complete fiction, leaving viewers walking away thinking everything they saw was completely true.