It's not about...
Occasionally, a film is not about what it appears to be about. (Or "a film is not about that about which it appears to be." See Grammar pedantry)
This trope originated with 2011's Tinker Tailor Soldier Spy, which is not, according to Mark Kermode, about spies. It is about men who do not trust each other.
Jaws is not about a shark. It is about infidelity (at least, the book is.)
The Towering Inferno is not about a building that is on fire - it is a morality tale, as indeed is The Poseidon Adventure; in both, those who have behaved immorally die, while the virtuous are spared from the respective catastrophes.
District 9 isn't about aliens; it's about racism.
Starship Troopers isn't about bugs; it's about fascism.
Doubt isn't about paedophilia, it's about Iraq.
The Hurt Locker isn't about Iraq, it's about men.
Interestingly, as far back as 2006 Sigourney Weaver was making the point that her film Snowcake was "not about autism", despite that being a key part of her character.
- Except there might be - in The Good, The Bad And The Multiplex, Mark says that "while A Night To Remember is about Englishness in crisis, and is therefore interesting, Titanic is about Hollywood in hysterics, and is therefore annoying." So hey - maybe there is some depth (arf) there after all.
Father Ted is not about the Irish church; Dad's Army is not about the Home Guard.
Mark often talks about how in 2001: 2001: A Space Odyssey Stanley Kubrick goes from the dawn of man to the birth of a new species in two hours 41 minutes. His problem with the film however is that it was written by someone for whom science fiction wasn't about allegories, it was about trying to extrapolate literally what would happen if human beings came into contact with alien life.
For a further discussion of why Tinker Tailor Soldier Spy is not about spying, see show of 30 September 2011.