The void at the top of the tank is a common thing in a lot of vehicles. The filler comes into the side of the tank leaving an area above it to act as a vapor zone.

I doubt that in cold weather that you could get enough turbulence during the fill process to get fuel to fill that zone. At least, not without rocking the vehicle and "burping" the tank of its extra air.

It is all interesting though. I don't plan on ever putting more than 16 gallons in again!