You may have hit on something. When I got my car ready for its first drink, I poured in 5 gallons and expected to see the gauge at 1/2 full. It wasn't. Maybe that first gallon is unmeasured.
Chuck, I have the definitive solution to your problem. We install the shiny one on my car and you put the old rattling one back on your car. The other solution (albeit not beneficial to me) is to use it as is and not allow people to look under the car.