They were still doing some CGI on the film when the trailers were released. There are also scenes in the trailers that never appeared on the final cut. They will probably be explained on the special edition dvd release. I know from experience (with video game rendering anyway) that rendering outdoor lighting that needs to bounce off of several different types of materials is a difficult process when trying to make it photo-realistic. Some lighting angles don't work well with the mesh in some cases, which makes it look less realistic and obviously cgi.