Why do some movies show up as SD, and some as 480P

Plex Media Server Metadata & Adding Files
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I’ve seen this. It’s the width which plex uses. 720 will show up as SD whereas 852 will show up as 480p.

BTW… Similar thread here Android App reporting 480p incorrectly as 576p were you can see a 480 height comes into plex as 576.