Well, when you choose 50% of course your fuel gauge in cockpit shows 75%. This is because it shows the amount of fuel in your MAIN tank.
But a F4U1 has more than one tank. 75% in main and 0% in wing tanks means a total of 50%. (Toggle shift-f to switch through different tanks)
If you select 75% fuel load in hangar, the main tank will have 100% and each wing tank about 30%.
I just checked E6B to make sure, and on my computer 50% resulted in 180gal, 75% had 272gal and 100% 361gal. So I canīt find a bug here.