I think its just a lot of people that move in a similar direction- at some point, they cross at bikes. With a crowd as large as SF, you're going to find an example of every kind of cyclist, and just because you catch someone riding a cheap, beat-up fixed-gear and wearing tight jeans and a messenger bag one evening, doesn't mean he doesn't have a suit-and-tie job as well, or that fixed-gear riding is all they do.

Even in a scene as small as Sacramento's, there is a wide variety of ages, amounts of time cycling, income levels, etc.