Is is on the basis of isotopic frequency as @Transform2942@lemmy.ml mentioned in their comment.

Since an element’s isotopes have different atomic masses, scientists may also determine the relative atomic mass—sometimes called the atomic weight—for an element. The relative atomic mass is an average of the atomic masses of all the different isotopes in a sample, with each isotope's contribution to the average determined by how big a fraction of the sample it makes up.

read more here: KhanAcademy