Quite simply, the gauge is wrong. Out of calibration.
LPG tanks (and tanks for other pressurized liquids) are filled to 75-80% of WATER CAPACITY. For the purposes of measuring propane, that is FULL. This is what a properly functioning and calibrated float gauge will say. But float gauges are frequently wrong.
LPG tannks (as opposed to DOT portable bottles) are usually filled with the bleeder valve open. The bleeder valve spits liquid when the liquid reaches the FULL level, which is 80% of water capacity. Tanks also have a overfill-protection device on the fill port, which might shut off before the bleeder valve spits. With these two protections, overfilling is unlikely.
There is no way a pump could fill liquid beyond water capacity of the tank, as the hydrostatic pressure would either go higher than what the pump could supply, or the pressure would go high enough to blow out the pressure safety valve, which is the third protective device on a LPG tank.
Tom Test
Itasca Spirit 29B