It's the same figure which was in the leaked documents from 2008 that points to some NSA/CSSM 1-52 document from 2007 as a basis. Perhaps they feel that this is the only justifiable piece of "statistics" they can provide at the moment. Though since the document was apparently OK to relay to foreign nations, this is likely another case of the "least untruthful" thing can say.
Reference 1st and 28th pages of: http://www.theguardian.com/world/interactive/2013/jul/31/nsa...