That's really counter intuitive isn't it?
For what it's worth here a my guesses.
Are you sure that they are not talking about a percent -- ie San Fran has the biggest percent of FFs of any city population. This actually does make sense then because all the listed cities are pretty affluent and highly educated.
My other (kinda off the wall) answer is that the cities listed (with the exception of West Palm Beach -- talk about a biazarre answer to what city has the most FFs) Is they are major second tier cities of business and people travel from those cities to NYC, LA, etc more offen than people from LA, NYC, etc travel to San Fran, Seattle and so forth