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Why can the atomic weight of natural lead vary from one source to another?

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What are isotopes and how do they work? — isotope An overview of isotopes.

The atomic weight of natural lead varies from one source to another because lead is the end product of the radioactive decay of heavier elements, specifically uranium, thorium, and actinium.1

While lead has four stable isotopes—lead-204, lead-206, lead-207, and lead-208—the latter three are radiogenic, meaning they are continuously produced by the decay of these radioactive progenitors. Because the isotopic composition of a lead sample depends on the age of the mineral or rock and the original ratio of uranium and thorium to lead within that environment, the relative abundance of these isotopes differs depending on the source. Since the atomic weight of an element is calculated as the average weight of its naturally occurring isotopes, these variations in isotopic distribution result in different atomic weights for lead samples from different origins.123