Perhaps (and I may be very wide of the mark here) the temperature of the fuel in the lines themselves (and therefore through the fuel flow meter) is an additional factor in all this?
Would cooler/denser fuel passing a gauging device show as a slightly reduced rate on the gauge? It is still the same 'amount' of fuel for combustion so the stoichiometric ratio would be maintained
I guess it depends if the gauging device is located on the fuel line in a warm spot (near engine) or at the tank.
I guess it also depends on how the particular gauging device measures the actual fuel flow too? The mass of fuel passing through the line doesnt change but the volume would?
My ignorance is showing
It certainly wouldnt be the major contributor but could be an additional factor.