A substance that is advertised as 100% pure can still have a somewhat lower (or even higher!) purity in the certificate of analysis like, for example, 99% pure in one batch and 99.8% pure in another batch. That doesn't necessarily mean that the substance is not 100% pure, but just reflects the biological or technical variation. With any analytical method, there is always the possibility for a biological random error and it may well be that measurements using different methods, different devices, or even the same method performed at different times can result in somewhat different results. A biological or methodological variation of +/- 2% is perfectly normal.