Why do the atomic mass numbers on older versions of the Periodic Table of the Elements often differ significantly from those on current versions?

Older versions of the Periodic Table often have different Atomic Mass numbers due to the historical methods used for determining atomic masses, which were based on less precise measurements and the relative abundance of isotopes. Advances in experimental techniques and a better understanding of isotopic composition have led to more accurate values, reflecting the weighted average of an element's isotopes as they occur naturally. Additionally, the introduction of standardization practices, such as using the atomic mass unit (amu) based on the carbon-12 isotope, has contributed to these discrepancies.