Because it runs on the same hostname as www.postgresql.org, and otherwise there would be a potential conflict. The folder /media/ is in the filesystem, and should be served up as /media-static/ on http.
What am I supposed to do to resolve 404 I am getting for these urls?
As I said, you need to make the folder /media/ be served up as /media-static/. This is configuration in your webserver.
What do you mean "present in the github" and the one that's in pgarchives? Where is it on github and where do you see it in pgarchives?
The CSS that is being loaded for this page: https://www.postgresql.org/list/ (actual CSS that pgarchives using) I compared this by inspecting components in the browser.