Haha, well judging from your picture I think the biggest culprit are all the hills that you're on. I've got about the same deal and if i didn't have any highway trips I would be averaging around 11-14. Thankfully my biweekly 100 mile 60mph trip gets that back up to 22 average (ish - i guess 21.0 as you can see below).

actually the only hills i go up and down are down my block and up the next block to get to the main street. i live in jersey city, not too many hills, especially the routes i take. i used to get about 17-18 in the summer when i just bought the car. thats all city driving, i rarely go outside jersey city. then it slowly started going downhill.