The "devil is in the details' and the problem with your findings is with the word "mostly". When you combine city and highway driving at all, as well as cold weather vs. hot weather driving, the results are 'all over the map' : meaning not statistically significant.
You didn't mention whether your car is an automatic or a manual. But if it is an automatic, then your results are typical and normal. Which means there is nothing wrong with you or the car - and the car IS what it IS, and not subject to change.
I have a scan gauge in my car ('09) since it was new. Having a scan gauge is one way to get highly reliable MPG data. In warm weather it will get 40 MPG in exclusive highway use, if all city driving is excluded (meaning driving at constant speeds of over 50 MPH with no stopping, until refueling). Unfortunately our car is usually used 98% of the time in local commuter traffic, at speeds of under 40 MPH. In winter, with cold operation and typical short trips of under 10 miles we can never get much better than 22 MPG, no matter how hard we try. That's nature, that's winter, that's traffic - all unavoidable. Anything in-between or any combination of highway-city is a blend of factors that doesn't really tell us anything meaningful - because it is much too variable.
Once you leave the highway, (as well as with winter driving in general) your MPG goes into the dumper. That's the Aveo experience.