The duke/duchess thing is weird, could be for some word trick in korean that makes it a gender neutral term that doesn't really work in english.
Tbf it's actually accurate in a way. A Duke is the leader of a dukedom, while a Duchess is the
Duke's wife. Note that the Duchess is
not considered the "leader" of a dukedom, because the title of Duchess has normally and historically been used in relation to the Duke, not the dukedom, and carries very little power in of itself. The Duchess is only a person of power so long as she is married to the Duke, but the Duke will always be a person of power even if he's deposed.
You can see this particular style of titles almost everywhere. For example, in Malaysia, "Dato'" is the honorary title akin to a knighthood, and is considered gender-neutral (e.g. the famous actress/signer Dato' Siti Nurhaliza). However, the
wife of the Dato' is called "Datin". Datin itself is not a title that carries any power but rather, one that shows relation to the Dato'.