For many cities, not just NYC and SF, but yes, it is true only for major cities.
EDIT: embellishment due to baq, this tends to be true of cities which developed before the car revolution of the 50's and 60's. For example, my city has a pretty poor average walkability score, but for where I live in the city core, it is very walkable since it is the older part of the city.
EDIT: embellishment due to baq, this tends to be true of cities which developed before the car revolution of the 50's and 60's. For example, my city has a pretty poor average walkability score, but for where I live in the city core, it is very walkable since it is the older part of the city.