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I believe I can answer this. What seems to be happening is when you are coasting with the engine on, the ECU is commanding the evap purge solenoid closed. This raises the vapor pressure in the tank and forces the float to sink slightly. I have seen this condition in vehicles with faulty purge solenoids, most notably with digital readout fuel gauges. I couldn't confirm this without consulting a manual for your truck, although you could verify it with a scanner while driving.
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