Can anyone explain my mpg experience

I am a fairly new owner of a very lightly used '02 Prius - had 7200 miles on it when I bought it about six weeks ago. It is my understanding that the EPA ratings on the car are about 52 City and 45 Highway - not as good as the new ones, but not exactly bad.

When I first got the car I drove it some locally and then went on a trip of about 150 miles doing 55-75 mph with a/c on, then came home and did more suburban driving. When I hit 505 miles I filled it and achieved roughly 54 mpg. I generally seem to be getting an average of 50-52 as my wife and daughter seem to know how to crater the gas mileage. My daughter has been successful at getting below 40 when she tries

Yesterday I drove from NW NJ to NYC on the highway at 55-70 mph with air on, a little driving around the city and then home and was able to get 55mpg.

My question is why is it that I actually seem to get better mileage on the highway than both the EPA ratings and slightly better than I get local. Although I must tell you that locally I have many big hills so I imagine that costs me in gas mileage as it takes more energy to climb a hill than you save going down.

 

I am talking about the MFD display here, but I have calculated it also and it comes out to +/- 1 mpg of what the display shows for each tank.

 

Thanks

I was wondering if this is normal from others experience or am I just lucky enough to have learned how to drive it just right quickly.