It is generally accepted that single engine Cessnas have unreliable fuel gauges.
The fuel gauges would have been reliable when first certified. So what happened in between? Easy question. Over time various things could cause fuel gauges to become inaccurate or faulty. You would think responsible pilots would immediately write up the suspected defect in the approved maintenance document.
But often they don't because they either can't be bothered or are afraid to make perceived waves and upset the boss. So they simply leave it to the next unsuspecting pilot to fly that aircraft to wear the problem (F**k you, Jack - I'm OK) Repeat ad nauseam. The worst sort of back-stabbing to the next pilot.
Often, flying school managers avoid encouraging clients to write up any snags because the aircraft owners can get snarly at the extra cost of fixing things. Parking brakes on Cessna's not working properly, jamming fuel valves on C152's, binding or jamming primer pumps, inoperative landing and taxi lights. They all play up occasionally and should be rectified but unless a pilot reports these on paper they don't get fixed. And that includes intermittent defects as above, including fuel gauges..