In my experience the big influencing factors are, in order:

1) Weather
2) Driving Style
3) Traffic / Route (city vs. hightway)
4) Car condition (tire pressure, etc.)

You'll read a lot about all these here. Just keep in mind - rain and cold hurt FE (Fuel Economy). New tires (in my opinion) do not help. Look for them to become less sticky between 500-1000 miles. I believe that one common trait we all had with our cars is that it took some time to adjust to everything. You'll be surprised at how damaging some of your old habits will be. Takes time to get used to a different driving style. Give it time - I wouldn't read anything into your FE data for the first 1K miles. Enjoy your new car!

*** Retired after 65,000 outstanding miles ***