You failed to note that the relative hilliness of two cities could have a huge effect on the reliability of using the "fixie index" to measure relative hipsterness. Even if they were equally hipsterish, it would stand to reason that LA would have more fixies than SF, since the bike is far more practical in that city.
On the other hand, Los Angeles is ten times bigger in terms of total area, and has less than half the population density of San Francisco. That makes it more practical to have a fixie in SF, where you're generally traveling shorter distances.
You can get almost anywhere a hipster would want to go in sf without worrying about the hills. It's easy to go around them for the most part, and barring a few exceptions, mostly low-density residential on the hills themselves.
