Sometimes during a single shot, there are important parts of the composition on opposite sides of the screen.
Often, by computer, the 'camera' continuously moves back and forth across the widescreen image in an attempt to keep the most important visuals in view.
This often brings an unnatural look to the film. Usually when real panning is done during the actual filming of a movie, there will be a subtle motion blur that is non-existant when the panning is computerized.