I snagged the map from James Fallows. Here's his explanation:
That won't come as any surprise to anyone living in contemporary San Francisco. The city's population is churning madly in response to healthy tech companies enabling their workers to use the city as a bedroom community.
I do wonder though about that magenta rectangle in the Mission. Were densities so high among immigrant and other low wage workers in that sector (think apartments where single rooms were rented to whole families) that upscaling the neighborhood reduces population? Perhaps.
Here's the same mapping technique applied to the whole country, again from Fallows. Anyone interested can do what I've done above at his post, zeroing on an area of particular interest.