Thanks Intruder,
Yes, I have handled quite a number of fire hoses, for exercise, in my time, as well as many, many overwing refuelling hoses - Caribou, Vampire, Canberra, C-47/DC-3, Neptune, F-86, Mirage, during my many years as a Ground Engineer, prior to starting flying!
No system, as far as I am aware, fully fills the available tank space with fuel, all leave varying amounts of airspace for thermal expansion, THAT'S WHY the overwing filling access is never right at the top of the tank, because then you would be able to completely fill the tank, and the designers don't want that!
I appreciate that the 'Pie-cart', as the hydrant truck is referred to where I come from, only dispenses, regulates and measures the fuel into the aircraft, and the fuel pressure and flow rate is increased because the system is 'closed' and is therefore much less liable to spill, exept for shut-off malfunctions in the aircraft fuel tanks.
Cheers