This gets back to what I said earlier in this thread. Based on my market observations, I believe traditional radio usage in San Francisco is lower than a lot of other places. Especially among younger people. Same with Seattle. If you look at the stations people listen to, the Top 10 is different from other major markets.
You make an interesting point.
I looked at market cume rating for LA, SF, HOU and NYC for 18-34 for the book that came out Tuesday and the prior two books.
New York 72.8 with weekly TSL of 4:41
LA 76.4 with weekly TSL of 5:41
SF 70.6 with weekly TSL of 4.52
HOU 76.0 with weekly TSL of 5:34
If you go to 25-34, it jumps up to just under 80. And on 25-54, it goes to near the mid 80's, but on the low side.
New York always has lower TSL because so many people use public transit and can't listen to radio in many or maybe most cases.
Which leaves SF on the low side with no reason except market uniqueness. I suspect part of this... and maybe all of the uniqueness... is in the nearly 30% Asian ethnicity of the market. Nielsen has no differential treatment of Asians, has no recruiting procedure for any Asian language and, thus, likely severely undermeasures that group due to language issues.
Seattle has a simpler explanation: it is the whitest of all those markets, and it can be shown that Blacks and Hispanics, for a variety of reasons, use radio more than non-Hispanic whites. Fewer Blacks and Hispanics, lower usage. We see that in places like Milwaukee and Fargo and Minneapolis, too.