Two different questions
HPLC purity answers: of the things the detector can see, how much is the target peptide? Net peptide content answers: of everything you weighed, how much is peptide?
What else is in the powder
Synthetic peptides are purified as salts. Charged parts of the peptide carry counter-ions such as trifluoroacetate (TFA) or acetate, and freeze-dried powder also holds some water. Neither shows up as an impurity peak on HPLC, but both add weight.
That is why a vial labelled 10 mg of powder can hold less than 10 mg of actual peptide, even at 99% purity.
How net content is measured
Labs measure it separately, usually by amino acid analysis or by nitrogen (elemental) analysis. A certificate that reports purity, identity and quantity per vial gives the fullest picture.
Educational information only. Nothing here is medical advice, dosing guidance or a recommendation to use any substance. Research peptides are not approved medicines and are not for human or animal consumption. Speak to a qualified professional and follow the law where you are.
