The real reason is
It is not advised to 'brim' off the tank especially in the hot summer. Reason being is that gasoline is stored underground and at a temperature a lot cooler than at the surface. Now if you pump 18 gallons of gas (petrol) and it is 25 degrees cooler than the outside air, it will expand. If you only drive a short distance and park, it will expand enough to overflow. If you park over hot asphalt the tank will also absorb more heat. If you drive a longer distance and burn fuel faster than the expansion rate, you won't overflow! Hope this makes sense.