I cannot explain, but we have a similar situtation. We drive 3-1/2 hours to MSP fly to CLT, spend the night and on to MBJ the next morning.
The ticket for that right now is $650, if we were to take the same flight only from MSP to CLT and back that would be $542.
The ticket for the same flight from CLT to MBJ and back is $612.
The people making only the one flight must be making up the difference to allow us to fly for "ONLY" $650. I can't complain too much.