I always book roundtrip, too. The one way internet price is for a particular leg of the roundtrip.
BTW, today I had to change one of my flights because there is a sale going on. The sale is through June 15th, which is my departure day. I was very concerned about changing the flight time as I thought the entire roundtrip would be affected. Not the case for me. Went in to change my reservation and they had boxes in front of both flights asking me to choose which I'd like to change. In the future, if and when they post sale fares for my return flight I'll do the same without worrying about any changes to the departing flight.
Hope this helps!