Why I ask for the gradient before I discuss the number

Confirming post #2 from a second method, which matters more than confirming it from a second person.

Gradient delay volume differs between instruments and shifts the whole chromatogram. It is why a transferred method rarely reproduces retention times exactly on a different system.

It is a small point and it changes the answer, which is an awkward combination.

Peaks that do not elute do not appear in the area percentage. Aggregates and strongly retained species can be entirely invisible to a standard method, which is a ceiling on what any purity figure can claim.

Post #10 and I disagree about the size of the effect, not about the direction.

A shallow gradient resolves close-eluting species that a steep one merges. Two honest laboratories running different gradients can report genuinely different purities on the same vial.

Adding the measurement that post #23 says would settle it.

Injection volume matters because column overload distorts peak shape, and an overloaded main peak can swallow a small neighbour. A certificate without injection volume is missing something load-bearing.

It is the kind of thing that is obvious once and never again.

Retention time is only comparable within a laboratory on a given method. Quoting a retention time across two reports as evidence of identity is not a valid comparison.

Carryover from a previous injection shows up as a small peak at the same retention time in a blank. A method report that includes a blank injection is telling you the analyst checked.

Take the reasoning and check the arithmetic; I do not always get it right.