krujje is right, of course, the energy is related to the mass, not the weight. Take a kilogram mass of fuel to the moon, and it will only weigh one-sixth, but have the same energy. Even on earth a kilogram mass does not weigh exactly a kilogram everywhere.
Torstennnn is right too, of course, in that the difference here on earth is so small that for our purposes we can neglect it. A kilogram on the scales is a kilogram mass, near as d@mn it
What's more, the energy per kilogram mass depends on the exact composition of the fuel, and the differences there are such, that the minute difference in weight becomes a second-order effect.
ITCZ,
Indeed, a capacitance gauge only measures fuel level (and by implication quantity). Specific gravity of the fuel is measured by a separate sensor (think of a float that sinks into the fuel more or less deeply, depending on the s.g.)
Fuel weight is then quantity x s.g.
"Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane."
Not only for uplift as such, but also for calculating the C.G.!
CJ