This has come up before on a discussion re: fuel quantity gauging using capacitance sensors.
I too, previously believed that fuel capacitance gauges measured conductivity of all the fuel in the tanks.
Apparently this is not so. A capacitance gauge is still a measure of level, not mass.
I'll search for the TechLog discussion. It was less than 6 months ago.
Kg/lbs are primarily used to have common units for uplift of fuel, pax, bags and aeroplane.