It's not vented at atmosphereic pressure. In fact, it takes quite a bit of pressure to "vent" the tank via the gas cap. The closer the tank is to empty the more you will notice the pressure when you remove the cap. Gasoline has fairly high vapor pressure so when you are running low it creates a lot of pressure in the tank especially when you have a hot day. When the tank is nearly full with liquid you won't notice it so much.
Years before this there was a vent pipe that just ran to the outside dumping all those vapors in to the atmosphere. Those cars did not use vented caps. My 69 uses a vented cap but it takes a few pounds to open it up. This was done to limit the amount of vapors escaping. Beginning in 1970 you start to see EEC systems (evaporative emission control) appear on all cars. That system does not use a vented cap or tank and forces all the vapors to a activated charcoal cannister for storage and then re-introduction in to the intake manifold.
Today, if your gas cap vents at all your "service engine soon" light comes on. A loose gas cap on a modern car will turn on the light. That's how far we have come in controlling evaporative emmisions.
