Consider the following three fill-ups:
|Jan 1||1500 mi.||25 gal.||Yes|
|Jan 15||1950 mi.||20 gal.||Yes|
|Jan 25||2505 mi.||28 gal.||Yes|
Between Jan 1 and Jan 15, you travelled 450 miles (1950 - 1500) and consumed 20 gallons of fuel (the amount of fuel it took to re-fill the tank). Therefore, your fuel economy for that time period is 450/20 = 22.5 MPG.
Similarly, between Jan 15 and Jan 25 you travelled 555 miles (2505 - 1950) and consumed 28 gallons of fuel. Your fuel economy for that time period is 555/28 = 19.7 MPG.
This calculation requires that you fill your tank. When you don't fill your tank the software will not know exactly how much fuel you've consumed since the last fill.