That is another good question I do not have the answer too. I'm guessing it's either :

- The standard density for Jet A, Jet A1 and Jet B fuels with error margins (Jet A1 is usually 0.800 Kg/L standard density, Jet A is 0.820) I do not know the density of Jet B, but its freezing point is significantly lower than the Jet A fuels, could that be because of a lower density (0.73) ?

- Ground temperature. 0.80 being density a 15°C, 0.73 would be 30°C and 0.82 0°C or something similar...

FSX uses Jet A-1 fuel at  6.7lbs/US. Gal and 0.8Kg/L.

Cheers

Nico