Why are the terms Eastern Balts and Western Balts sometimes inaccurate?

The traditional classification of Baltic tribes into "Eastern" and "Western" groups is often considered inaccurate because it fails to account for the entire scope of the Baltic-speaking population.1

Specifically, these terms are problematic when applied to all Baltic groups, as they exclude the so-called "D-Balts" (or Dnieper Balts). These tribes resided in the upper reaches of the Dnieper River and were assimilated by Slavic populations between the 7th and 14th centuries. Because these groups left no written records, they are frequently omitted from the traditional binary categorization that focuses primarily on better-documented tribes like the Lithuanians, Latvians, and Old Prussians.21

Furthermore, the classification is complicated by the nature of individual languages; for instance, Curonian is sometimes viewed as a transitional language between the East and West Baltic groups, rather than fitting neatly into one category or the other.3