This is not all together true. Usually trailer shots are called out at some point in the visual effects schedule of a show, and those shots are finished first specifically for the trailer deadline. The shots are not discarded and rarely "different" from what appears in movies, but can frequently be revised. In this case it is easy to see that the pterodactyl shot was a plate that could be used as a POV shot in earlier commercials and now has more of the shot done (and is likely considered to be finished, or labeled CBB - could be better / couldn't be bothered).