When you setting the order of the images in a gallery you are telling the system what you think is the "best order" for them to be displayed. This "order" is used but the system will take the sizes/shapes of images into consideration, making slight changes to their positions for an optimal aesthetic view.
So, images that are ALL the same size… it will become more apparent that the images are not in the exact order… but images of varying sizes will not be noticed because things will look good visually.