Perhaps your area recently did the change up between summer blend and winter blend gasoline. However, I thought summer blend garnished the greater mpg gain.
http://s184.photobucket.com/user/kac...g.html?filters[user]=48335090&filters[recent]=1&sort=1&o=0
That fog light area already looks sealed up, meaning an air pocket forms of which the actual air flow goes over and around.
I don't get the mpg gains either, other than as people have mentioned that there is a cumulative affect.