Although the charcoal canister is several feet away from the tank and even several inches above the top of the tank if the car is overfilled and on an incline so that the fuel is entering the vent line when the system is in the vent mode vacuum will draw the liquid upward. Fuel can become trapped in a low section of the vent hoses where it will be eventually drawn into the canister. When you consider the shape of a tank, the location of the filler neck and the slope where the car is parked when filling up it is more easily recognized that some people scoff at the idea that stuffing fuel in causes a problem because they have been doing so for decades with no problem. They might be quite $urprised one day when they fill up at a different station where the slope positions their car to overfill. A nearby self serve can totally blow any effort to keep track of fuel mileage. If I enter from the street my mileage is 15 +/- but if I fill facing the street it is nearly 20. And the slope is not very noticeable.