B777Heavy
I know exactly what you mean and have been pondering this for a while. During the climb till not long after, fuel used plus fuel remaining is a couple hundred more kg than that departed with. I am guessing that this is the result of a guage overread during the climb. This does resolve itself not long after top of climb with fuel used plus fuel remaining equaling the departure fuel less any small divergence due to APU burn (if it is running) UNTILL that last 1000kg of centre tank fuel is used. Once the centre tank is depleted, the sum of fuel and that remaining is a figure 2-300kg shy of what you departed with. This relationship continues right through to shutdown.
If you land with say 9000kg and haven't touched that last 1000 kg or so of centretank fuel this apparent loss of fuel does not occur.
I have seen some documentation from Boeing advising that the reason is the combination of fuel flow transmitter inaccuracy, FQIS reading high in full tank conditions and low at lower fuel levels and fuel remaining in the centretank when pumps are turned off. Apparantly it is all within limits!!!????
If you load on an extra couple of hundred extra kilos of fuel for flights that arrive with the centre tank depleted you seem to arrive with what you should.
B777, that 800kg could well be a good 300kg from what I describe above plus 5 hours of APU burn (say 250kg) plus a bit of rounding at each end.
cheers