I very much doubt the topper has much to do with it. At most I think you would be getting 1/2 mpg variation for wind resistance.

Mileage can vary dramatically depending on many factors such as wind direction relative to travel (if your going 70 against a 50 mph wind, its just like doing 140, and going the other way would be like doing 20)

In such a short distance, relying on the computer, I find mine reads higher near the top of the tank, and lower mid tank.

Load, speed compared to your average, hills, stops and starts, etc all can affect your reading.

18-19 mpg unloaded 60-65 mph with a few starts and stops thrown in seems to be fairly normal.