Here's a very particular picture of Ann Arbor, MI. How many downtowns does it have? (If you're from there, you're excused from this little quiz.)
Here's a couple of close-ups of the center of that smaller green blob off to the side:
That last picture is the central intersection in this "walkable" area (Jackson and Maple).
If you aren't familiar with the W Stadium corridor (what we're looking at), that probably wasn't quite what you were expecting to see. (But maybe it was if you noticed that particular swatch of green contains a mothafuckin' highway.) Huge setbacks, infrequent crosswalks, very wide intersections with high-speed turning radii... the urban design of the area is not terribly conducive to walking, which makes it hard to call it walkable.
Here's a particularly potent example. Grocery stores are a pretty important amenity, right? I might well be wrong, but (commuting aside) I'd wager that, for most people, if they had to choose for one thing to be easily walkable from where they live, it'd be a nice-sized grocery store (y'know, smaller than a Walmart, bigger than a bodega). The Kroger in the area is located here, just a scant few blocks from the major intersection pictured above:
Here's what it looks like if you're trying to walk up to the store from the sidewalk:
Notice that there's no clear pedestrian path from the sidewalk. You have a choice between contending with the near-constant rush of cars as you walk on the blacktop of the entrance/exit or walking through the carved out bioswales on either side (which are filled with tall grass, animals and insects that inhabit tall grass, and all manner of litter). Neither are very welcoming for the traveler on foot.
The bus stop - if you can call it that, the sign doesn't have schedules or even route numbers - is nested between two such entrances.
It gets worse. Note that the store is set so far back you can't even see the sign from the street. A pedestrian has to walk through at least 450 feet of vehicular traffic to get to the front door. Should you need to catch the bus on the other side of the street, good luck. This stretch of S Maple is just shy of 4 tenths of a mile long, contains a staggering 33 curb cuts - on average, one curb cut every 60 feet - and outlets on major thoroughfares on both ends. This is not a quiet street. Looking for a crosswalk? There are two - one at each end.
I wouldn't call that a walkable urban place; it doesn't exactly provide for a pleasant user experience. In fact, this particular area seems to have some features specifically designed to disincentivize walking; not having pedestrian paths leading from a storefront up to the sidewalk is a dead giveaway.
And yet, Walkscore gives this area a score of "71 - very walkable." So what's going on here? It's hard to say for sure as their algorithm is proprietary, but based on what they will say their major measure of walkability is density of amenities. The more dense and varied a collection of amenities is, the more likely the walkability is high, so sayeth walkscore.
Here's another wrinkle: what are these amenities supposed to be accessible from? As you can see from this Ann Arbor Population Density map (City-Data.com), the areas with high Walkscores are only kinda/sorta correlated with where people live. For instance, the heart of Downtown Ann Arbor (south of the Main / Huron intersection) does happen to feel very walkable, but only once you're there. (Several high rise developments have gone in recently and more are coming, so that's going to get less true as time goes on, thankfully.)
As you can see, Walkscore doesn't always give the most complete picture. Urban design and proximity of housing/office space can matter just as much as accessibility when it comes to a given area affording not owning a car.
Why does all this matter? Mostly because Walkscore loves to promote research that says stuff like this:
"One point increase in Walk Score was associated with between a $700 and $3,000 increase in home values."
-Joe Cortright, Impresa, Inc. (2009) Walking the Walk, CEOs for Cities
I don't doubt that this is true in the aggregate, but it clearly is not true universally. If rental and real estate agencies are using this information to increase the amount of money they're asking from you based off the raw Walkscore alone, you might well be getting fleeced a little. If you're in the market for a condo or an apartment that advertises a high walk score, you might want to use Google Streetview to check out an area and make sure the premium they're asking for is actually worth it.
Don't get me wrong - I love WalkScore. The information they give you is definitely better than nothing. And, to Walkscore's credit, they are very aware of the flaws in their methodology and will even tell you about them (if you click around enough). The fact that they have a proprietary product called "Pedestrian Friendliness" - which is apparently different from what a WalkScore is! - shows that they're self-aware about this. I just wanted you to be aware as well.