There is a difference between the Preview Image Width and the general image width that is also found in image files. For more information about the general width, click here: Image Width
Unlike plain image width, the Preview Image Width can be found in all kinds of files like videos, documents or e-books. As long as the file has an embedded preview or thumbnail image, the value can be found there. Thus, Preview Image Width is not associated with the actual image dimension, but the ones of said preview image.