The comment about municipal boundaries is really apt. Cambridge is 6.4 sq mi with only a few areas that are less dense. Boston's score gets dragged down by places like Hyde Park and West Roxbury. New York City's score gets dragged down by Staten Island and Queens.

It's hard to really compare places because what "feels like" the same place and municipal boundaries don't always overlap. It's why people say "Camberville" since it often feels like the same place and distinct from Boston which requires crossing the river.

In Los Angeles, there are parts of the city that are separated from the rest of the city by 5-7 miles of mountains. It would be like considering Nahant or Hull part of Boston since it's only separated by a few miles of water. Likewise, how many New Yorkers have never set foot in Staten Island?

I think a lot of people in Boston have never been to Hyde Park or West Roxbury simply because they don't feel like a part of their city - not to exclude them, it's just that they feel apart from a lot of Boston in the way Staten Island feels apart from Manhattan or the Valley feels apart from the rest of LA. There are all sorts of informal boundaries of where people go and where they don't which aren't the same as municipal boundaries.

Even in Cambridge, scores vary. On Brattle St, it can be down in the 60s: https://www.walkscore.com/score/159-brattle-st-cambridge-ma-02138. In Central, it can be slightly higher at 97: https://www.walkscore.com/score/619-massachusetts-ave-cambridge-ma-02139. I don't tend to go to West Cambridge much because it feels a bit apart from the part of Camberville where myself, friends, and the shops I go to exist.

Still, while it's hard to compare places, Cambridge is a really walkable place. Manhattan would probably score higher if it were evaluated separately from NYC as a whole and that's probably why Cambridge and SF beat NYC - NYC is just much larger and includes some more suburban places as a result. Even then, the intense walkability of Manhattan really buoys the score for NYC.

EDIT: Oh, and Somerville gets an 89: https://www.walkscore.com/MA/Somerville. So the Camberville area is really walkable.