A weighted average of what?
"The isotopic mass for each isotope, that is, the relative mass of each isotope on the
126C scale." p. 127,
Intro to Chemical Principles, by Stoker.
"A relative scale of atomic masses has been set up....The unit is called the atomic mass unit, abbreviated amu. The arbitrary reference point, involves a particular isotope of carbon,
126C. The mass of this isotope is set at 12.00000 amu. the masses of all other atoms are then determined relative to that of
126C.
Since isotopes exists, the mass of an atom of a specific element can have one of several values.... Despite mass variances among isotopes, the atoms of an element are treated as if they all had a single common mass. the common mass value used ia
weight average mass[italics the author's], which takes into account the natural abundances and atomic masses of the isotopes of an element." pp. 126-127.
Therefore, the "isotopic mass" is a relative mass not a weighted average. My mistake.