According to the site,
Neighborhoods scores are initially calculated using demographic data from the US Census and other public sources. Wide numbers of demographic factors, such as age, marital status, whether the household has kids or not and the frequency of arts and culture activities, among other variables, were compared. The scores are then refined over time by reviews from people just like you.Could be, I'm not so sure about the DC vs Manhattan though - it says Columbia Heights is like the Financial District, which is preposterous, and Harlem and Yorkville, which seems a little more accurate. For DC vs SF, it says Columbia Heights is like the Mission, which is reasonable.
The cool thing is, you can edit the ratings, as I did for Columbia Heights vs Financial District.