Gas gauges on most boats are highly inaccurate and are really only for relative or approximate guesstimates. Not like a car which people rely on to know how many miles until really empty.
You give the engineers way more credit than I do. Usually the sender is selected by the depth of the tank, where by default, float all the way down = Empty and all the way up is full. No calibration, etc. With the shape of the tanks in most boats, the number of gallons in the tank vary with the levels as well.
Thatís also why most boating courses provide the 1/3 rule for fuel. 1/3 out, 1/3 back and 1/3 in reserve. Iím not referring to 1/3 of a tank, but the fuel needed to do a specific trip.
To accurately know whatís really in the tank, use your a log book. I log each trip and fuel up, so knowing my average burn rate, I can calculate gallons remaining after each run. My math should sync fairly closely with the gauge, but when in doubt, I go with my calculation, not the gauge.