shallow experience with a few 07 cars - about gas mileage

In the past month or so, I rented different 07 cars on weekly basis, on purpose to feel how they perform. I drove virtually all local (highway with high traffic liked local anyway), and paid particular attention to gas mileage of each. Here is what I found:

1) Camry, I4 - smooth, more power than 01's, flittle noisier than 01's,still felt floating, ~21 mpg

2) Outback, V6 - fast pickup, muscular, hard suspensions, choppy feeling, road noise heavy, ~20 mpg

3) Forester, I4 - fast pickup, smoother than Outback, noisier than Camry, overall better than Outback. ~21 mpg

4) Altima, I4 - powerful,fast pickup, heavy feeling, stable, road noise level between Camry and Outback, 22 mpg

5) Impala, V6 - smooth, quiet, heavy feeling, stable, good performance, ~23 mpg

6) Sonata V6 - smooth, quiet, stable, pickup not as fast as Forester but good performance in general, ~21 mpg

7) Milan (Mercury) I4 - road noise heavy, choppy, engine seemed working hard, average performance, ~24 mpg

It had been in my mind for years that asian cars were gas saver, interestingly in real world I found somehow american cars at least above two models had better gas mileage. Was I wrong? Can somebody explain this?