Verizon, the phone company whose disclosure of customer data to the federal government is at the center of the furor over cooperation by technology companies with top-secret national security programs, has offered a precise, clear, but little-noticed public explanation of why it did what it did. The Verizon explanation is not in the vague and cryptic memo the company issued last week after the Guardian exposed its program. It came, instead, in the company’s annual filing with the Securities and Exchange Commission, included in Verizon’s annual report to shareholders.