Fuel measured by volume
Very interesting about the VNav.
My only addition is that all fuel systems, be it the ultrasonic FQIS, the capacitance fuel guages or even sticking the tanks, actually generate a depth of fuel at a given point. When everything is working and the aircraft is level, it is straightforward to work out a volume. Obviously, if probes are misreading, or if you are manoeuvring, it becomes more complex.
Obviously, you need to multiply by a density (usually around 0.8Kg/lit) to get a mass. If you have a 50T load, clearly, it only takes a slight change in density (which will obviously change as the temperature changes) to create a significant difference in fuel remaining.
The one advantage that the ultrasonic system has proved to have, is that it is much better at getting a density than other systems.
When you measure fuel flow, that will measure the volume pumped to the engines. I don't know what is used to get density in this system.