How do you calculate your mpg? The way I do it to have the trip meter running, when I go to fill up with gas, I record the mileage since my last fill up (then reset), divide that by the amount of fuel put in and get the mpg (the gas station receipt has the amount of fuel sold). If you are using an app, it could be a problem with the algorithm's assumptions, or some other issue.
This is exactly how I calculate mileage. I have a little cardboard slide rule that BP was giving away, probably 20 yrs ago and it calculates the mileage this way. Funny thing is my wife got it when she had her 1989 Mazda 626.