Second pass at: What a reversed-phase purity number actually is

On the subject in the title: Second pass at: What a reversed-phase purity number actually is Working notes rather than a conclusion.

I would like to understand what this number means before I repeat it anywhere.

A VendorInvestigate report on a semaglutide lot gives 97.5% purity. The supplier certificate for the same lot states 97.8%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

Coming back to the opening post, because the follow-up matters more than the original answer.

Area percent is not mass percent. It is a proportion of absorbance, weighted by each species' extinction coefficient. For closely related impurities the approximation is usually good. For structurally dissimilar impurities it can be poor.

I disagree with the reply above, and I think the disagreement is substantive rather than terminological.

The distinction being drawn does not survive when you look at the published data for this specific question. I would be glad to be shown wrong on this, because the version I am arguing against is more convenient.