If most of your trips are very short, this will dramatically effect mileage. Also, don't rely on the computer. Compute the mileage by dividing the miles driven by the number of gallons it takes to refill the take.
My F150 pickup is rated for 15 in town. Most of my trips are only 1-2 miles in length. The truck actually is getting between 11-12.
My Jeep Wrangler was rated at 18 in town. It was a 6cyl sport. Making the same short 1-2 miles trips, it got 13 in town. But making 15-20 mile average trips, it got 18 which was what it was rated for.
Gas mileage depends on a lot of variables. The distance, how hard you gas it at red lights, (I'm bad for this one) and the amount of stop and go you do. Those posted mileage ratings are an average. If you look at the sticker, it will post the range.
RickC