The Department of City Planning and Department of Buildings both certified the project for 709 feet.
Maybe the issue is structural height/roof height??
If you look at the renderings, the roof appears to be different based on if you are measuring from the perimeter or at the center:
http://www.thecityreview.com/lex610.html
There are many such issues, such as with the Herzog & DeMeuron tower in Tribeca. The construction guys say 825 feet; the permits say 796 feet.
From CityRealty:
Last night, Community Board 5 narrowly passed a resolution supporting the transfer of air rights from the iconic Seagram Building, at 375 Park Avenue, to a new development at 610 Lexington Avenue (the transparent building in the photo of the developer's model).
RFR Realty LLC is the owner of both the Seagram Building and the property to be developed, currently home to to a YWCA that will be demolished to make way for the new 257,000-square-foot, 62-story building, which will be 709 feet high.