Lots of variables here to consider. I noticed OP didn't list all specs on his truck. (Sorry if it came later & I missed it but it's 4:05 am LOL). Things to consider OP...
1. Miles on truck matter. It's not fully broken in until you reach about 15k.
2. Rear gear ratios, tire selection, tire pressure, trans, driving habits, altitude, fuel mixture (NOT to be confused with octane rating), wind speed/direction, vehicle weight, radio station selection, political affiliation and the gravitational pull of the moon all matter. Some more than others.
3. Hand calculation at fill-ups vs the Lie-O-Meter in the dashboard (it is always optimmstic).
If you were really optimistic about mileage, you wouldn't have bought a brick. LOL (I Hope).