Movies posters generally don't represent a real scene in the movie, but are staged for a reason. That could be to relay who the actors are, or to set a tone. I'm not sure how trailers are different. I think the problem is that you believe something about them that is not true, which is that they are made up entirely of scenes from the movie. Historically that may have been the case, but it was almost purely because that was the easiest and cheapest way to get a high quality trailer. We've moved past that now, and it's possible to easily represent the feel of the movie using custom material in trailers, like it's always been for movie posters.