Here's another stretch. According to the "Atlantic Yards Chronology" (p. 6 of this PDF):
Forest City Ratner Companies ("FCRC"), the developer of the Project, has already acquired approximately 85% of the project site.
The word "acquired" covers a lot of ground. On the developer's own web site, the FAQ states:
With recent acquisitions included, FCRC now owns or controls 86 percent of the land needed for Atlantic Yards.
What's the difference?
Here's a December 2006 property ownership map from the Empire State Development Corporation, which shows that the Metropolitan Transportation Authority railyard and city streets are not owned by the developer.
Also, remember that the developer once claimed 90% before a court said that property it claimed to control (Lots 5, 6, & 13 on Block 1129) was improperly leased. That case is under appeal. More background here on the difference between ownership and control.
In other words, the project isn't quite as far along as claimed.