Hi Paul,
I don't think there's anything wrong with your car. The instrument pack has circuitry which processes the signal from the level senders and takes into account road speed; effectively it introduces an element of current fuel consumption into the gauge indication. My guess is that this is re-calibrated when the ignition is turned on. Since you left the engine running when refilling, the instrument pack didn't know that extra fuel had been added.
Thanks for carrying out that useful experiment!
Simon
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Sir Henry Royce.
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