It is a real world issue that GA fuel gauges are highly inaccurate, often giving indications up to a quarter tank misreading.
14 CFR 23 describes the certification standards of airplanes. Sounds like you’re aware of this already. However, the “long-historied” airplanes in the GA world are governed by a mix of CAR 3 and 14 CFR Part 23 (as components get modernized).
From a manufacturing standpoint, these instruments are indeed calibrated during completions. However, once they get into real world, deviations happen.
This is why we have procedures as specified in 14 CFR Part 91 that requires that we do flight planning and validate fill-up in Preflight - External checklist with visual inspection prior to engine start. From there, we need to make sure that we’re rotating tanks (single feed planes) and that our indicator is “roughly” in line with what we’re expecting. Something wildly off can be an indication of another problem.
The indication is a mechanical tool that has its own faults. The mechanical bob within the tank on a GA airplane is not precise enough for your engine to run out of fuel at exactly the 0 indication.