Why do people quote purity to two decimal places?

I am comparing a supplier certificate against an independent result on the same lot.

The empirical answer seems settled. The explanation does not.

If the honest answer is that nobody knows, I would rather hear that than a plausible story.

What is the causal chain, and where does it stop being established?

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.