Splitting 100LL fumes

When I filled the fuel tanks for the very first time on my slowbuild RV-6A, I was impressed with two items. #1--- The tanks did not leak. #2--- The tanks hold more fuel than I would have supposed. Each tank, upon initial fill up accepted 19.9 gallons of 100LL. Thats 39.8 gallons total. Has anyone else noted this phenomena? I would reasonably guess that by placarding each tank as holding 19 gallons, the designer intended we have a significant "unusable" amount of fuel in each tank...almost a gallon.

Rick Galati RV-6A "Darla"

Good point. I used a store bought container clearly marked with a one gallon line. I filled the container over and over to precisely that line. This was done so that I could initially calibrate the installed FL-2C fuel level gauge.

Rick Galati RV-6A "Darla"