The thing is that the UV island displays correctly when the aspect ratio of your image is squared. Usually your textures are 512×512 or 1080×1080 or 2048×2048 and in those aspect ratios the UV islands displays correctly. But then if you load an image that is 1920×1080 and it’s more rectangular and you load it up in the UV editor, the UV’s will stretch to fit that “weird” aspect ratio they are not used not. Try it out and see for yourself.
