National Security Letters can be issued to obtain information about a subscriber from telephone and Internet companies. The FBI has the authority to prohibit companies from talking about these requests. Google has been trying to find a way to provide more information about the NSLs we get. We’re reporting numerical ranges rather than exact numbers. This is to address concerns raised by the FBI, Justice Department and other agencies that releasing exact numbers might reveal information about investigations. We plan to update these figures annually. We’re thankful to U.S. government officials for working with them to provide greater insight.”]
Source: https://publicpolicy.googleblog.com/2013/03/transparency-report-shedding-more-light.html