If you look at the concentration of blue in the Bay Area, you can see that they need to add a city to "What is 'the City'?":
http://www.businessinsider.com/22-maps-that-show-the-deepest...
Hint: It's not San José.
http://www.businessinsider.com/22-maps-that-show-the-deepest...
Hint: It's not San José.
It probably has to do with the density gradient. With more spread out urban areas having less distinction between "the city" and outlying areas.