Not to harp on the SF ridership (OK, I'm harping), but looking at these maps of car-free households and rail lines, I'm still dumbfounded that SF has twice the ridership of Chicago.
https://www.liberallandscape.org/202...united-states/
Chicago has a vastly larger geography of car-free living. And it isn't just low-income tracts. Many multiples larger. And the same is true of Boston, DC, Philly and even Baltimore. And SF almost certainly has the worst transit quality of Chicago-SF-Boston-Philly-DC.
But that said, I have no reason to doubt the data. It just must be that car-owning Bay Area households use transit much moreso than in other transit-oriented metros.