Oil Tank Not Full But Whistle Sounded

Had a fuel delivery today. Got a call saying the guy could only pump like 20 gallons before the whistle went off indicating it was full. However, there is no way possible the tank is full because we have like a gallon tank and there is only like 200 in there. Would check the gauges but they don't work (guy coming to fix next week). I am highly concerned. Why would the whistle go off if the tank is not full?
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