In some areas of the country, the gas nozzle is just a narrow metal pipe that fits loosely into the filler opening. Gas fumes that have collected on top of the fuel's surface are pushed out of the tank, into the air, when the tank fills with fuel and the fumes are displaced.
But in many other areas of the country, gas nozzles have a contraption that sits over the entire filler opening -- it's meant to catch those gas fumes and suck them back up into the station's tank, thus preventing the escape of gas fumes into the atmosphere.

The trouble is that sometimes this contraption doesn't work well, and it causes pressure to build up that falsely triggers the filler mechanism (valve) to shut off. This is especially likely if the car's tank's own vent is also clogged, so pressure cannot be relieved. Thus, the tank resists being completely filled.