Peptide content is the proportion of a lyophilized solid's mass that is actually peptide. The remainder is counter-ions and bound water, and it is not a rounding error: synthesised peptides are typically isolated as salts, commonly acetate or trifluoroacetate, and the counter-ion contributes real mass.
It is distinct from purity, and the two are easy to conflate. A sample can be 99% pure — meaning almost all of the peptide present is the intended peptide — while being appreciably less than 99% peptide by weight. Purity describes the composition of the peptide fraction; peptide content describes how much of the vial that fraction is.
The practical consequence is that a vial's nominal mass and the actual peptide it contains are different numbers. Where a calculation has to be exact, take peptide content from the certificate of analysis for that specific lot rather than from the label.