Ralph is right. The computer is making approximations.

The only way to accurately calculate a change in MPG per tank is to use the number at the gas pump. Even if your odometer reads higher or lower, your % change in MPG will be accurate.

Just recently my HiHy read 40.4 mpg over the first 10 miles after fill-up. I can assure you that at my next fill up, my calculated mpg will be around my usual 21mpg.