The Korea surcharge is both for after midnight and out of Seoul, both 20%. BTW, in Seoul, the "Foreigner" taxis (orange) have the language(s) spoken by the driver on the door. If not, translation is available by saying "translation" and the driver will call a speaker of that language that you can talk to on their mobile phone. I don't know why he would turn it off nearing the airport. Fare was about right amount.

As to Seoul vs. HK; I have had fewer problems in Korea than Hong Kong with taxis (I speak some Korean and Chinese, but I don't look like it) so drivers sometimes feel that can get away with indirect routing.