Counterion
In short A counterion is a charged particle that stays attached to a peptide after synthesis and balances its charge. In chemically synthesised peptides it is usually trifluoroacetate (TFA).
The counterion comes from purification and adds to the weight. That is why the mass in the vial is greater than the mass of the pure peptide; the difference shows up in the peptide content.
Some certificates state the counterion content separately, others only the peptide content. Both describe the same thing from different directions, and both can be checked in the certificate library.
How much counterion a molecule carries depends on its sequence: it attaches to the basic side chains, that is arginine, lysine and histidine. For sequences with few basic residues about five per cent of the mass is typical; for strongly basic ones it can reach twenty-five.
It follows that the proportion is not a mark of quality. A high trifluoroacetate content is not a defect but a property of the molecule and of the purification. It only becomes relevant if trifluoroacetate interferes with the planned experiment; in that case there are lots with acetate as the counterion.
In practice this means that the nominal amount on the label is an upper limit for the quantity of peptide it contains. The article what is really in the vial works through an example.