Some years back the fuel pump sucked out the gasoline and as air was somehow stopped from taking up the space used by the gas used the tank collapsed and only a little fuel could be put back into the tank.
I have forgotten most of the details.
There is a flap that stops you overfilling the tank and that is why the fuel spills out the filler hole. The gas should not be able to fill above the level of the overfill flap.