You explained it in your post, city driving and stop and go traffic or slow traffic on urban expressways .
The engine is running but you are not moving any appreciable distance during that time .
Sitting still, is '0' miles to the gallon .
I have 1.5 miles surface streets, then urban expressway, 65 mph, with my work schedule, I leave the house between 0330 and 0515 and leave work at 1230 to 1430, so I miss 'rush hour' .