If you're comparing steady-state 70 miles per hour to steady-state lower speeds, you're bound to get worse mileage, all things otherwise equal. (See 1stpik's post.) The Prius MPG Simulator
verifies this. (It calculates the effect of speed and many other variables on MPG.)
On the other hand, your post suggested something other than steady-state. You may be (somewhat intuitively) keeping the vehicle's RPM in efficient operating ranges by allowing speed to increase on the downhills and to bleed off on uphills. Some advocate a specific technique called warp stealth
on hilly highways to improve mileage at highway speeds. I have tested this on an interstate highway with gentle hills, comparing WS at an average speed of 60 MPH
to allowing cruise control-maintained 60. (Results not posted .... One of these days I'll finally
get the last edits completed!) With those identical average speeds, the results were nearly identical. WS might still be of some benefit on terrain with steeper hills. Hobbit (the author of the linked WS paper) certainly believes it.
I notice you just recently bought your Prius, so presumably your highway driving in it is still rather limited. You can't judge your results on such a limited experience. Many other factors affect fuel economy, including weather. Temperature and winds, in particular, can have a significant impact. A cross or head wind as little as 5 MPH
, for example, can drop it by 8%.