David, it's not strange, there
is no problem and there is no confusion amongst pilots.
Helicopter fuel gauges are often calibrated in Kgs because as pilots we have to calculate the aircraft AUM as the greater priority. The SG of fuel changes very little, in the big picture. I quoted an example to show how little a different SG affects the contents of the tanks.
If we wanted it to be
really difficult we could calculate passengers weights by the litre and their exact SG, too.
As long as the gauges show when there is fuel in the tanks and I have a conversion factor for their calibration I couldn't really care if it says 500 litres, 500 Kgs, or 500 bananas on the scale.