I'm not sure about the earlier posts in this topic, but the Turbine Duke has different fuel capacities in each wing. Since the MSFS payload interface sliders load the aircraft to a given percentage of fuel, I would expect this difference to show up if you load the aircraft with the sliders. The left fuel system has a capacity of 138.4 gallons and the right has 132.5, which would account for your 6 gallon difference if the sliders were set to 100% for both tanks.