Why were those discs sent?

The fact that this data was being sent anywhere is a matter of concern but I learn from The Daily Brute that they were being sent to the National Audit Office. Why?

The NAO's own web site states its job is "Helping the nation spend wisely" and goes on:

The National Audit Office scrutinises public spending on behalf of Parliament. We are totally independent of Government. The NAO is headed by the Comptroller and Auditor General, Sir John Bourn.

We audit the accounts of all central government departments and agencies, as well as a wide range of other public bodies, and report to Parliament on the economy, efficiency and effectiveness with which they have used public money. Our work saves the taxpayer millions of pounds every year.

Why on earth do they need the details of 25 m people? If it is for fraud then that is a bloody big sample to be trawling through. If it was to test the Revenue's processes then they have proved a point, but did it need 25m entries to be sent as some sort of test.

I am genuinely perplexed and would welcome any thoughts on this issue.