There may be a federal standard for how gas pumps are supposed to work, but I have personally seen a difference between pumps, sometimes at the same station... maybe their shutoff triggers are slightly different or aged, one pump is used more, etc. At one station I visited, the closest pump to the entrance rang up 19 gallons in a car with only a 15.7 gallon tank. I stopped getting gas there. When measuring gas mileage, your best bet is to use something like a ScanGauge, or fill the car at the same exact pump 5 times and average out the readings over time. Plus, your first tank isn't enough to judge MPG... drive it more, it will probably improve.