Never noticed that before. But it looks like they are just filling as much of the viewport with your image as possible for each aspect ratio, rather than have a black border all the way around your image.

I personally never apply arbitrary frames to images, since every image is different and it would be a great coincidence if the best cropping for the dynamics and composition of an image just happend to match some arbitray aspect ratio rule. Of course i'm not making images for TV or any other required industry standard aspect ratio.

_jc
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