That's a very good and very pertinent question ...
Taking a wild guess, and assuming that the aircraft was being loaded, given that the 777 Freighter has the Main Cargo Door on the rear part of the fuselage, which is very useful to avoid "wheelies" on the ramp, they loaded the main deck forward of the main cargo door, then they loaded the "interesting cargo" in the rear ... and did not had time to load the last cargo, which is the one that goes in front of the Cargo Door itself.