My first inclination is to believe that the weight & balance data was correct as those processes are normally highly automated and highly accurate
... as long as the correct data are entered ...
According to the METAR there was a slight headwind. A ten-hour flight is not a long flight for this aircraft so it will not have been near the MTOW which, depending upon the MTOW "purchased" by the airline, is around 372k kg's
Headwind ?
Also 6300 NM would be more around 14 hours flight ...
My estimate would be around 310 tonnes (no cargo) ... (?)