Car computers are notoriously inaccurate. I have observed errors up to 12% in vehicles from multiple manufacturers. Car computers can be helpful in a relative sense to tell you whether one style of driving is more or less efficient than another but they're basically worthless for absolute calculations. The only reliable way to accurately calculate fuel economy (in mpg units) is to divide the number of miles since you last refueled by the number of gallons to fill the tank.