http://essay.utwente.nl/61212/1/Jong...verslag%29.pdf
Long. the upshot: as has been theorized and explored many times, people choke under pressure. The more emotion and fear are present, the worse they do. This applies to golfers, police officers, and you and me. It is the rare person who actually overcomes extreme pressure and emotion to do a movie-worthy performance in the worst of situations. It's certainly not that people don't want to do well, they just, often, can't.
ETA: I do agree that people can be trained in ways that reduce this. This does NOT apply to the general population no matter how skilled they are at target shooting, it's specific mental training and it's hard enough to accomplish in military and law enforcement individuals specifically drilled about it. While some people, for all I know Tradd herself, may be naturally less prone to "choke", one never knows.