Regarding the database mileage difference, I have to point out how drastic the mileage varies based on demographic. I am a Camry Hybrid owner living in Massachusetts. I consider myself very skilled at driving hybrids for best fuel efficiency. In the summer, my tank averages are always over 40MPG, sometimes reaching 43MPG. In the winter, my mileage plummets down to 33MPG on the coldest weeks. That's over 20% difference between best and worst MPG. Also, in cold winter states, the formulation of the gasoline is changed for half the year to reduce emissions during engine warm up. This altered formulation has more than 5% less energy density than normal gasoline.

Also, Toyota's 4cyl engines are tuned more for efficiency and less for performance compared to Nissan. As posted by others, this would impact fuel economy.

Regarding the Altima mechanicals, Toyota worked very closely with Nissan on altering their gas engine to work best with the hybrid system. I'm pretty sure this includes Atkinson technology (or something very similar). If the Altima engine used the Otto cycle (with little or no modifications), I doubt it would get even close to such high highway mileage.

 

The 2.5S/SL Altimas with a CVT get 31mpg highway, so a little detuning to 158hp (opposed to 175), a bit of hybrid battery help, and an eCVT could definitely make up 2mpg on the highway. The Altima uses the exact same 2.5 that the non-hybrids do, but it's detuned for gas mileage. It's not an Atkinson cycle like the Prius or Camry.

 

I have a TCH that I drive in the North (RI) in the Summer and South Florida in the Winter. In the Summer I have averaged between 42 and 45 mile per gallon per tank. In Florida in the winter I average only about 37 to 39 mpg per tank. I think a lot of the variation is just caused by the environment that we drive in. In RI I live in sort of country area so I drive much slower (two lane country roads etc.) and in Fla I am driving much faster due to higher local speed limits and sometimes longer trips (Ft Myers to Miami etc.)