This one has a reasonably settled answer, so here it is. Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself calibrated, the gradient and column, the detection wavelength, and where the integration baseline was drawn. A percentage point or two between competent labs is expected. What is not expected is a large gap, and the usual explanation there is that one report is not of the batch in your hand.

Dr.GutHealth said:

Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…

No disagreement with Dr.GutHealth. One condition attached. One addition: an unstated method makes a report close to uninterpretable. "99% pure" without the technique, the standard and the conditions is a claim, not a measurement.

zoe_NC said:

Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.

Can confirm the pattern zoe_NC describes. They are not contradictory, they measure different things, and this is the single most common misreading of a certificate. Purity is a relative area measurement: the main peak as a percentage of total peak area in the chromatogram. Content is an absolute mass measurement against a reference standard. A vial can be 99% pure and contain 6.1mg — pure material, overfilled — or 99% pure and contain 3mg, which is pure material and a short fill. Purity tells you what fraction of what is there is the right molecule; content tells you how much of it there is. You need both, and a certificate with only purity is telling you half the story.

From the other side of the consultation, briefly.

My HPLC chromatogram analysis for certificates of analysis — I sent my batch to Janoshik and here's what the results showed:

Main peak retention time: 16.3 minutes (consistent with semaglutide reference standard)
Peak purity: 98.2%
Related substances: 0.5% (single impurity at 10.0 minutes — likely a synthesis intermediate)
Mass spec: m/z 4113.5 (matches expected molecular weight)

This is a clean result. The mass spec confirmation is particularly important — it proves the molecule is actually semaglutide and not a substitution.