Would someone be able to give me some insight into how the vent from the fuel tank is suppose to work?

In the process of troubleshooting some of my fuel problems, I was double checking that the vent line from the fuel tank was clear. The vent line from the fuel tank (under the ski locker) goes up to the fuel filler neck where it has a separate little hole next to the fuel fill hole. I took the line off from the tank and blew through it (toward the gas cap) to make sure there where no obstructions. It seemed to be fine. Obviously when the tank is decreasing in fuel, air would be coming the opposite direction of my test so there wouldn't be a vacuum in the tank. What I don't understand is how the vent line can pull air when the gas cap is on since it's located under the gas cap. It seems like a tight fit and there's no visible way for air to be pulled in through the cap itself. On my last boat the fuel vent had a separate port right next to the gas cap. Dirt-dobbers like to build there nest in there and that was another problem. Any enlightenment would be appreciated. I want to make sure I'm not getting a vacuum in my tank when the engine is running.

2006 Outback DD Carb