Bottoming Out Our Suspension of Disbelief

Why is it that sometimes we can accept the most unbelievable situations in a movie, but not believe in the little things? For example, in Alien 3, a bullet goes through a window as a ship is entering Earth's atmosphere. My roommate at the time thought that the movie was sooo fake, cause there's no way that the ship could maintain it's altitude after that kind of change in cabin pressure. It's funny that he had no problem with the fact that an ALIEN that was birthed in somebody's stomach and burst out then grew to full grown size in mere moments got sucked out the window in the process, or the fact that they were flying a spaceship in the first place.