In my non turbo legacy I average 27 highway and 24 city, epa estimates for my car are 20 city / 27 highway / 22 combined.
EPA estimates for your car are 19 city / 24 highway / 21 combined.
So I honestly don't think there is any issue. I think the first few weeks were a fluke, and you are now averaging what you should be. Colder weather/winter blend gasoline could be a contributing factor as well.