On your car the vent line should be going down under the car and ending there. Check for that when it's on the lift. If it doesn't end there and indeed does end in the filler tube you need to fix that.
I think I understand you. I guess I assumed the vent line came from the tank to the top of the filler tube to allow air trapped in the tank to escape as it fills (in other words, connected to the tank on the bottom and filler tube on top). It sounds like you are saying the that vent line is not connected at the tank, only at the top of the filler, so it can vent off gas and fumes below the trunk floor.
If that is the case, I suppose turning the car could get the fuel to slosh up the filler tube, and with nowhere to go, it would flow through the vent tube and dump below, causing the heavy smell and occasional spill that I have seen.