Blue you are mistaken.
75% fuel loadout in the F4U-1 fills the main tank, however unlike all the other F4U's it also has wing tanks.
So 100% fuel fills all 3 tanks.
75% fuel fills the main tank and also gives 25% fuel in the wing tanks.
50% fuel shows as 75% in the main tank.
I also just checked the drop tank scenario you posted and it still shows as correct.