Why 99.2% and 97.8% on the same lot can both be correct

Why 99.2% and 97.8% on the same lot can both be correct I have a specific reason for asking rather than idle curiosity, and the context is below.

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

A Janoshik report on a retatrutide lot gives 98% purity. The supplier certificate for the same lot states 98.8%. 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?

Practical note that does not fit anywhere else. Whatever you conclude from this topic, write down what you did and when. The single most useful thing in your own records is not any individual result; it is that they are dated and consecutive.

Method validation is the demonstration that a method can separate the compound from its degradation products and impurities reliably. A method that cannot resolve an impurity from the parent peak will not detect that impurity.

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods and they will not produce the same number.