There are a number of factors that affect the Prius MPG. Just like other vehicles, driving conditions, operator actions, vehicle condition, including the engine, and so on.
The Prius Hybrid design efficiency, where the engine only runs under specific conditions, you notice little things that affect mpg more.
For example, you can have an MPG when you park the car and when you turn it on and and the engine warms up you see your mpg drop. In the colder months you see the mpg dropping fast to get to the warm up state. So if you only live s mile away from work your car is spending a lot of its time warming up your gas mileage is not that good. You get good gas milage with your Prius only once the engine is warm.
Of course your mpg is less in the winter since the cold air coming in the grill will drop the Prius engine temp faster and causes the engine to run more. Some Prius owners go as far as blocking off some of their grill openings to adjust for this. I don't as the Prius doesn't have an engine temperature gauge.
Running the passenger compartment heater also reduces gas milage as you are taking heat from the engine and the engine needs to run more to maintain a minimum temperature.
Well, I guess you get the point of how a high mpg vehicle like the Prius is impacted by these variables so I won't go on.
I noticed my mpg decreasing over the summer from my previous recordings and I surmised that it was due to my 12 volt battery being less efficient. For example I have easily gotten 56 mpg driving to my son's house in MD, but my trips were at 52 this summer. (No A/C on)
I changed my Prius 12 volt battery just before a trip again to MD last weekend and on the way down I got 54 mpg and that was with the heater on all the way down at about 44 degrees average outside temperature. I got about the same on the way home at about 55 degrees outside using the heater set at 72.
Then since I've been home I driven to the Boston area and I'm at 54.7 without heat.
I know this isn't a controlled scientific study but it appears that a Prius 12 volt battery that is older and is not as efficient as it used to be draws more power from the high voltage battery (how it is charged) thus requiring the engine to run just that more to make up for it and you see the difference in MPG.
Last year I got over 60 mpg from El Paso to Tucson on I10. It is with segments of my trip that I get high mpg that makes up for leaving my Prius in "Ready Mode" to run my fridge when I hike and still get 52 mpg average for the trip.