The flight plan that the crew had would have been computer generated with the route, it is all checked and the fuel on that would have been correct. Either the wrong number was written down and handed to the fuel guys or it was those pumping the fuel put in too much. The fuel figure written down and handed to the ground crew will be checked by both pilots. I expect they just put in too much fuel. Not the pilots fault, it does show on the display in the flight deck but you don't really pay attention to it that much as expect the number you give to be the amount loaded. If it was the wrong number written down then yes its the pilots fault but I think the case of just adding too much is more likely as fuel figures are very carefully cross checked by both pilots.