It all is how many stop lights, and driving stop and go. vs never having to stop.
If you reset your in dash mpg you can see the mpgs drop every stop light,and every time you have to reaccelerate. simple physics.
It take a lot of enegy to accelerate. having to do it over and over every few minute eats up an amazing amount of gas.
So does sitting idling. but not nearly as much as accelerating.
So say you have a city commute of three miles at 35 to 40mph with only one stop. Your mpg over that distance will be in the 38mpg range. The same distance at rush hour, with a stoplight every other block..Your mpg will be in the low 20's at best.
Same car. and person.
THAT is the only difference.
Yeah mashing the pedal makes it worse, but the real problem is the stopping and accelerating over and over.
Ther is no conspiracy, no magic involved. Just stopping and accelerating. is the problem.
I would also say that most cars I had driven this issue was never so clear as it is in the 3rd gen Focus.
Probably has to do with the high mpg which CAN be gained, vs the usual mpgs.. and the way the engine is designed etc.