Purity is a proportion, not an identity
High performance liquid chromatography separates a sample into its components and measures how much of the total each one represents. The headline percentage is the share of the sample made up by the largest peak.
That tells you how much of the material is one substance. On its own it does not tell you which substance that is. A very pure sample of the wrong compound still reports as very pure.
Identity is a question of mass
Mass spectrometry measures the mass to charge ratio of the molecules present, which acts as a fingerprint. Comparing the measured mass against the expected mass for a known sequence is what confirms the material is what the label says.
This is why certificates report identity as confirmed or not confirmed rather than as a percentage. It is a yes or no question about which molecule is present.
Reading the pair together
Taken together the two answer: this is the compound we expected, and it makes up this share of the sample. Either result alone leaves an obvious gap.
A certificate that reports only a purity figure is incomplete in a specific way, and worth asking about. So is one that confirms identity but never quantifies the material.