This surprised me too. Anyone from New York care to comment?
So while the data may show people leaving New York, it may not tell the whole picture. My own experience shows otherwise, though I am aware that anecdotes are not data. People may be moving out of New York City, but they aren't leaving New York City.
Here's an article from the same guys explaining the difference:
http://www.citylab.com/politics/2014/04/2-very-different-mig...
On top of that there is a lot of international money moving into New York, as another commenter mentioned, and generally into the 2+br apartments with a family, further driving up the cost of living even with a roommate in the "safe" areas.