[sp332]

receive a tax break for offering some units to low-income tenants

Literally the second sentence of the article. On top of that, units in the same building often cost vastly different amounts of money depending on the height, the view, etc.

[Grue3]

I was talking in general, not New York specifically. Of course if a third party (the government in this case) basically pays the difference, anything can happen. But it seems like it's because you can't just build new housing in New York City, so existing houses have to accomodate the poor, which is a pretty unique situation.