My guess will be due to the lower fuel tempreture after a long flight. As the 777 fuel sensor measured the quantity based on velocity of sound, with in turn is directly propotional to the density of the fuel. Therefore, with a colder fuel, this will results in a denser fuel thus, with the formula, density X volume = mass, the totaliser tends to "over read".
Just purely my guess, standing by to get corrected.
Cheers,
lion-g