Interestingly, I have read on this thread that this is a concrete core building, but that the ironworkers or the union insists that the core be built behind the steel. But I came across this, and it seems that the core following steel is done to < drum roll >
save money and time.
Here is the article. 7 WTC was also done the same way.
Unlike most New York City high-rises, the steel will lead core construction. This strategy was developed by Severud in the 1970s, but never implemented in New York until recently, says DePaola.
The construction manager selected the approach because frame construction is about one-third quicker, says Joseph L. Ross, executive vice president of Tishman Construction Corp. of New York. Tishman also is using the approach on Seven WTC and says the strategy relies on the use of climbing forms, which allow the core to keep pace with the steel.
http://enr.construction.com/Sp_Projects/green/archives/040726a.asp