A trailer is rarely produced by the people who made the film. It is commissioned from specialist agencies working to a marketing brief, which explains its most puzzling characteristics.
The brief is different from the film's
A film exists to tell a story over two hours, while a trailer exists to make an audience decide to attend within roughly two minutes.
Those are different problems, and the second is closer to advertising than to storytelling, requiring a distinct set of skills.
Agencies specialising in this work compete for the assignment, and studios often commission several to cut competing versions of the same trailer.
Editors work from unfinished material
Trailers are frequently assembled while the film is still being edited, using footage that has not yet been locked, graded or finished.
Shots that survive into the trailer may be cut from the film entirely, and effects seen in marketing may be at an earlier stage of completion.
This is the ordinary reason a trailer contains moments absent from the finished picture, rather than any attempt to mislead.
Music is licensed separately
A film's score is usually composed late, so it is unavailable when the first trailers are being cut.
Trailer music is therefore drawn from libraries built specifically for the purpose, structured around a slow build and a rhythmic final section.
That convention explains why trailers for very different films can sound remarkably similar, and why the music rarely appears in the film itself.
Structure follows a tested pattern
Most trailers establish the world, introduce the problem, accelerate through a montage, then break for a title card before a final beat.
The pattern persists because it reliably communicates genre and premise inside a short window, and audiences read it fluently without conscious effort.
Deviating from it is a deliberate choice, usually made where the film's appeal rests on a performer or a director rather than on the premise.
Approval sits with marketing
Cut versions are tested with audiences, and the results shape which material stays, how much of the plot is revealed and where the emphasis falls.
Filmmakers may have consultation rights, but the decision generally rests with the distributor, whose objective is attendance rather than fidelity.
That division of authority is the underlying reason a trailer can misrepresent a film's tone while remaining an entirely accurate piece of advertising.