The tank has an expansion hump which allows an area for the fuel to expand to on hot days when nearly full. When filled quickly air gets slightly compressed enough in the hump to "bloop" back out and thus also push some gas out when the pressure from filling eases. The only cure for gas vehicles is to slow down or stop filling for a second or two when you think you are nearing full to allow the air to escape before trying to fill to the brim.