Has anyone figured out the algorithm for how vk creates a preview for an image type=z?

Question

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The docks says https://vk.com/dev/objects/photo_sizes

z - proportional copy of the image with a maximum size of 1080x1024;

Loading a 1200 x 1137 photo, we get z = 1140 x 1080, wtf? why not 1080x1023?
Loading a 1380x1308 photo, we get z = 1139 x 1080, wtf? why not 1080x1024, is it ideal?
Loading a photo 2600x1602 we get z = 1280 x 788. wtf? here they just reduced the photo by half
Here you can poke api https://vk.com/dev/photos.get (check photo_sizes)

Answer 1

Perhaps the documentation is out of date. Should read:

z - proportional copy of the image with a maximum size of 1280x1080;