The indication is weight, but through the engines nozzles goes volume. Therefore as long as the tanks are full ( volume) irrespective of the weight being shown, you will get the range.
Now, everyone is "happier" when the weight is "higher" on cold fuel, leading to the view of higher range, alas...there is no such thing, like GF wrote above, at the end of the day after 10 hours at minus 50 ( to -70) you will get your fuel "back".
Some probes are compensated for temperatures some are not, i would believe that the 8X and 5X are, not on the F2TH or GLEX early generations
AFAIK