I've found that with KLM sometimes it's cheaper to buy a return than to buy a single. That one really had me confused.
Kind of like how buying a flight from Oslo to San Diego (BA) was 500-1000 USD cheaper than from London to San Diego, and the flight from Oslo basically just connected to the one in London (ie. same flight from London in exact same class). The people that price airline fares are from a different planet it seems sometimes