Considering that they're geographically approximately in the same area as countries like Poland, Belarus, and Ukraine, which are considered to be Eastern European, what makes them such a differing geopolitical group from the Baltic countries? How did the concept of "Baltic" separate from the concept of "Eastern European"?
They're not. Baltic states share similar history, but they're still considered Eastern European, much like Poland, Czechia and Slovakia are West Slavic while Belarus and Ukraine are Eastern Slavic.
While the Baltic states have considerable influence from Scandinavian countries (particularly Estonia with Finland), 'Baltic' and 'Eastern European' are not mutually exclusive.
First of all in Europe distance is a wrong way to measure similarities between countries. I'd say Estonia and Lithuania are more similar politicaly and culturaly to Czech Republic and Slovakia than to Belarus which is between them.
Besides Eastern Europe is misleading label, there are more differences than similarities between them. Belarus is authoritharian Slavic country with culture based on Orthodox Catholicism and then communism-atheism where USSR love still lives well. Estonia is Baltic democracy with culture based on protestantism and Scandinavian influences. Poland is Slavic democracy with culture based on Roman Catholicism and huge influences from Germany and Italy. Their views on modern politic, east-west dispute, freedom of individuals vs uniformity etc are all over the place. Their economies went completely different routes after the communism fall.
These countries main common point is being under USSR rule for 50-70 years before 1989-1991.