I seem to remember reading it's to do with fuel volume increasing/decreasing with temperature. Therefore loading 100 gals/ltrs of fuel will give you different volumes at different temperatures, whereas loading 100KGs will always give you the same amount for the purposes of calculating range/duration etc. (although the volume of 100KGs worth of fuel will vary with temp). I'm in no way shape or form an expert so stand to be corrected if that isn't the case
