You need to log the fuel bought and miles run over several fill ups to get a decent average. As the car is new to you perhaps you are giving it more beans to try it out.
The mpg on the dash can be +/- 10% out as it not based on through put of fuel , but many readings that are used.
You could also check to see if any of the brakes are binding on each wheel in turn.
Do you drive on your brakes or allow the car to slow down on its own ?
Consumption improves in the summer and goes down in the winter. Also depends on the mix of driving and length of trips, whether its garaged overnight.