I did some poking around and in version #9 it was set to 20 after Jeanette Sadik Khan defined it as a neighborhood slow zone.  This PDF url is provided: https://www.nyc.gov/html/dot/downloads/pdf/2012-06_inwood-neighborhood-slow-zone_cb12.pdf  On my next visit I'll be looking for those 20mph signs but before will be sending a note to DOT about why the NYPD doesn't know about this.