There are exact standards both for the accuracy of the reference material and the precision of the COA. So NIST can work backward, using faster tests to confir

There are exact standards both for the accuracy of the reference material and the precision of the COA. So NIST can work backward, using faster tests to confirm that the material has suitable qualities to represent, say, 2% milk, then take the precise measurements for the actual batch of sample, which of course will vary.