They're driven by different drivers. Most of the forum posts I've read suggest that the Honda drivers, on average, drive more conservatively than the Prius drivers. But all I know for certain is I got better mileage driving a Prius (I rented one for a couple of days, twice) than I did with the HCH. And I get slightly worse mileage with the TCH than the HCH, yet this car is 700 lbs more and I tend to drive a bit quicker in it too (and it's like 4-5 seconds quicker to 60 mph).