I suspect what you say is correct, however, I wasn't advocating using premium fuel in the Altima. I was suggesting -other- fuels may give better mileage, as reported by other hybrid owners (Prius owners mostly). Fuels such as Shell regular.
The bold statement that premium "could/should get less mileage" is a little presumptuous without a test, however, which is why I didn't say it.
The Altima appears to be running on the Atkinson cycle (though this isn't stated by Nissan anywhere I can find). I say this because the engine produces less HP than the tune level in the Altima 2.5s, is reported by Nissan at slightly higher compression (9.6 vs 9.5), but is persumably tuned for better mileage, all indicators of an Atkinson cycle (valve timing shift).
I -would- expect the Hybrid Altima to get about the same mileage as the Hybrid Camry, and so far Marianne is reporting that - about 38 MPG. I'm just surprised it does so without breakin having passed. Perhaps it will exceed the Camry. We can only hope!