it can happen because your display is scaling down a lot more information into the 1080p container. Depending on how the display handles it, you are effectively getting a super sampled image. We use this in film and tv production as a way to get a true 444 color image off a bayer pattern sensor (shooting 8k for a 4k delivery).
kinda a technical rabit hole to go down, but you can look into chroma subsampling here [https://en.wikipedia.org/wiki/Chroma_subsampling](https://en.wikipedia.org/wiki/Chroma_subsampling)
but when super sampling, basically you wind up with so many color samples regardless of your subsampling approach that the scaled down 422 video can become a 444 video. though this depends on how the service provider (say youtube) has transcoded the master files that were uploaded to them.
You also kinda get a sharpness boost when you oversample down to 2k or 1080. The early HD telecines like the spirit 4k leveraged this to their advantage when creating 1080p (or 720p, I think it had that mode) images for broadcast
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