re: "...I can't explain why your computer would read correctly while the gauge doesn't..."

The reason why the computer is accurate and the fuel gauge isn't is simply that they use two different methods of measurement.

The computer must use an electronic or electromechanical fuel-flow measurement device -- that's the only way that the nifty displays ("left stalk" controls on, e.g., my daughters new S60) can show instantaneous (as well as average) fuel economy figures. Most large powerboat owners have (or want) such a fuel flow meter, and these devices are incredibly accurate. Certainly, the "float level" system of traditional fuel gauges couldn't possibly have enough resolution to give this sort of information, moment-to-moment, that a fuel-flow sensor routinely provides.

The fuel gauge, of course, relies on the aforementioned traditional float design, and that's prone to mechanical errors (a leaking float, loose arm, dirt preventing free movement of the float arm, bad contact spots on the armature coil, etc.).