The hands of a king is the hands of a healer???

Ok... i ahve a question i've been wondering about since i saw the movies + read the books, which was around the same time.
In movie #1 when Frodo is stabbed by a Wraith, Aragorn clearly says: "This is beyond my skill of healing, he needs Elven medicine", but in the last book Aragorn can suddenly heal both Eowyn and Merry from the same type of wound, given the same plant, Kingsfoil. Why's that???
thanx if u answer this, cuz i can't really figure it out....