Average price/gallon doesn't consider number of gallons actually purchased
It appears that the "Avg. Price/Gallon" reported is just calculated by literally averaging the separate price-per-gallons logged for each fuel up. It would be better if it were calculated by dividing the total cost of all fuel ups by the total number of gallons purchased in all fuel ups. This matters if you buy more gas at low-price fuel-ups than high-price fuel-ups.
For an example of where this would make a difference, say I buy 1 gallon of gas at $4.00/gallon, then later buy 10 gallons of gas at $3.00/gallon. Fuelly will report the "Avg. Price/Gallon" as $3.50, but is should really be $34/11 = $3.09.
If someone buys fuel over several years, and fuel prices yo-yo as they have been doing, the average price is really pretty worthless. A more useful number would be the total amount spent on fuel for as long as the vehicle has been tracked.