Jet fuel is "sold" by volume
As has been pointed out, most aircraft buring Jet fuel do all the fuel calculations by mass, not volume - our aircraft fuel gauges are calibrated in pounds, the flow meters indicate pounds per hour, the FMS calculates pounds required, all our flight planning calculations are done in pounds, and when you work out what uplift you need, you calculate that in pounds.
But, when you get the invoice, it is always priced by volume. (For those not familiar, the international standard pricing measure is usually US cents per US gallon). So, when comparing prices, and deciding which leg to tanker fuel on, you have to take into account the fact that if the posted price is identical, the fuel at the hotter airport is actually more expensive!