I seem to recall terms like vapor pressure and thermo expansion and a bunch of other term that relate to gasoline. The common sense approach would be the gas comes from tanks below that is more than likely 70 degrees or so. It goes into your tank and it expands due to the temperature rise and the heat also causes a vapor to be produced. As the vapor excapes throught the cap, some condenses and falls out as a liquid. I think the trick is to not fill up all the way and after you have filled up, don't park in the direct sun until you have had time to burn off some of the gas. My two cents!
