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Why was this the case? Due to Bialystok's bare Polish majority combined with the fact that Stalin possibly wanted to give some sort of concession to the Polish Communists, however small and symbolic, in exchange for having them agree to the Curzon Line? Or due to some other reason(s)--and, if so, which reason(s)?
BTW, here is some interesting historical ethnic information and data about Bialystok Oblast:
Back then [as in, back in 1897], it was basically 34% Polish, 1/3 Russian (26% Belorussian, 7% Great Russian), and 28% Jew. It went to Poland after WW1. The USSR initially incorporated it back as the Belastok region, but eventually decided to let it merge back into Poland in 1944. (In fairness, by that time, it was 60%+ Polish).