When a batch of images are uploaded through the panel, they appear in alphabetical order in the files section. If the order is never changed through the panel, the images are never given a sort number.
If you then use
images->sortBy('sort', 'asc') in a template, the images rendered on the page do not match how they appear in the panel (I’m not sure exactly what the rendered order is from). This can be fixed by re-ordering any of the images in the panel, but that seems like a bug or unnecessary step for the end user.