Wind resistance. The lower the speed, the better mileage almost any car gets, to a point. In the city, the stop start kills the wind resistance advantage. If city driving is handled mostly on battery, better city economy makes sense. It might also be due to an underpowered conventional engine that's struggling at highway speeds.
I own a Hyundai Santa Fe and the best mileage I've ever recorded is when I spent several hours driving through Yellowstone Park. The speed limit is ~40 but with Summer traffic it was usually 30 with few stops... just small variations in speed. This vehicle normally struggles to get 23mpg but in the park, it achieved 31 mpg!
A long time ago I read a Popular Science article (?I think?) about what to do if you are low on gas and will run out before the next gas station. The article said to start the vehicle and gently accelerate to about 20. Shut off the engine and coast until it stops. Repeat. The article claimed even a gas guzzler could get crazy mileage with this technique (something like ~50mpg). Of course it will take you all day to get anywhere but in a desperate situation...