Greetings
Between the fuel meter and your aircraft fuel tank there is a manifold, which contains the 100 kg missing
, because when they stop refuelling the fuel has already been trought the meter, but will remain is the fuel tanker manifold as he is disconnecting the bowser from your aircraft
Some airline have agreement with tankers to provide the manifold capacity (100 kg, 50 kg..) and to substract that quantity from the meter
