When I go to load fuel in the fuel in the FMC, I was attempting to load 50500lbs of fuel on board. I would input the number in the FMC and when I went to press the button beside total fuel it would end up with the aircraft displaying it had 30200lbs on the in cockpit screen, but the total on the FMC was 320000lbs, very lost on why I am having the different read outs?