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.