I used to travel from BRU to the UK on regular basis and often found that a J fare was cheaper than a Y fare. When I got onboard I would often be the only J pax but Y would be full. All about demand.

Equally, a couple of years ago we wanted flights to MBJ and had to route LHR- MIA-MBJ. First was £1000 cheaper than Business. Very strange as F was full on both TATL sectors.