Flatlander "sweet spot"?

I'm in FL and have had my Prius for 7 months now, and it currently just turned to 8 K miles. I have noticed that now and then while driving around 55 to 62 MPH, I hit a "sweet spot" where the ICE is running and driving the wheels, and also charging the battery all while getting around 65 MPG. Nothing else is happening, just the ICE running and the battery charging. This continues for many miles, as long as the road is flat. I've purposely tried to induce this mode, many times with success. The throttle is applied gently, and the cruise is in the OFF position. Could the ICE possibly be getting this kind of mileage, or could it perhaps be something else? I really try to drive this thing for good mileage (but never below the speed limit) and have averaged around 52 to 53 MPG over the life of the car so far.

 

ScottY, that sounds pretty close. Thanks for showing me that post.