Actually, what does it is that pressure takes the path of least resistance.

If you fill your tank from a third line, the pressure in the tank has two ways to escape. It can push air out of the vent hole, or push fuel through the same size hole. It's much easier to push air through so that's where it escapes.

Now you MAY push some fuel into the line, but even if you do, it will be a slow trip all the way to the carb. And if, by chance the fuel makes it all the way to the needle valve, it will create even MORE resistance.

So you MIGHT get a little fuel in the carb, but certainly not enough to worry about.

And if you ARE getting a lot of fuel in tha carb, you need to examine your fuel system... There's a problem somewhere.