NW has had terrible one way prices for years, they just don't want to compete in that market for whatever reason. It was a major frustration of mine when I was flying NW more frequently several years ago since occassionally I only needed a one way and came home by car etc.
Within Skyteam both CO and DL usually have good priced one-ways (many routes are half of the round trip but not all of them). AA also offers good one way pricing most of the time as does US.