[rwg]

You probably want something like "en-US,en;q=0.9,ko;q=0.8". (Note the addition of "en" between "en-US" and "ko".) Some quick testing with Firefox, which lets you directly alter the Accept-Language: header in requests in about:config, shows that fedoraproject.org has "en" versions of resources but not "en-US" versions. Since your Accept-Language: header only lists "en-US", "ko" ends up being selected.

EDIT: I just noticed your guesses at the bottom of the post. Your second guess is correct. See §14.4 of RFC 2616:

As an example, users might assume that on selecting "en-gb", they will be served any kind of English document if British English is not available. A user agent might suggest in such a case to add "en" to get the best matching behavior.