This last report, which spans July to December 2012, contains vague data about National Security Letters. NSLs are a form of request for information that the FBI can make when they or other U.S. agencies are conducting national security investigations.
NSLs are an alternative to court ordered warrant and subpoena, and require only that the FBI director or another senior designee provides a written certification that proves that that the information requested is “relevant to an authorized investigation to protect against international terrorism or clandestine intelligence activities.”
Via NSLs, the FBI can request information such as the name, address, length of service, and local and long distance toll billing records of a subscriber, but cannot ask for things like Gmail content, search queries, YouTube videos or user IP addresses, as explained in Google's User Data Requests FAQ.
Also, the thing about NSLs is that their existence can be hidden from the investigated person. The FBI only has to write that the disclosure of the NSL may result in “a danger to the national security of the United States, interference with a criminal, counterterrorism, or counterintelligence investigation, interference with diplomatic relations, or danger to the life or physical safety of any person,” and Google (or any other provider) is forbidden to talk about the request.
This is why, in this latest Transparency Report, Google may only share the numerical range within which the actual number of NSLs they have received and the users/accounts they referred to rests.
In 2012 - and all years since 2009 except for 2010 - NSLs received by Google were between 0 and 999, and the users/accounts they applied to were between 1000 and 1999.