The airlines can probably explain this somehow but I don't see how adding a 4,000 mile round trip from Detroit to L.A. can lower a price.
Anomaly? Nope. I was flying from Newark to Greenville, SC. Fare was ~$650, connecting through Charlotte. If I flew only to Charlotte and drove the rest of the way? $970. Somehow adding a round trip to Greenville made the price go down.
To me, it seems the airlines price by what someone will pay, not what it costs the airline to operate. Business travelers and their routes get hammered.