As has been alluded to by previous posters, fuel density variation can be significant,
Ihave dropped 1500 imperial gallons of petrol into an underground tank, the diprodshowed 1530 Gal........12 hours later, it's at the correct .1500.
Diesel/Kero is pumped and metered, without density-correction, you get less WEIGHT of fuel on a warm day, than a cold one, if metering is by volume.
I find the use of "mixed units" (Kg / Lbs / Litres /Imp. Gals. / US Gals. ) quite disturbing....do the Bowsers have load-cells and deliver WEIGHT ? or metering-pumps and deliver by VOLUME ?
ISTR the Gimli incident was caused by just this "scrambled thinking"
Weight is, of course, the crucial issue with a small aircraft.
The arbitrary "nominal" weights used on a Pax. airliner probably give a wider variance of true load than the fuel density variation.
so, all pretty hypothetical, really, as far as big jets are concerned.
When it's a choice of fuel /luggage/a soul to keep within W&B limitations, that's a different kettle of fish!