I think southern Poles, Slovaks, western Ukrainians, Slovenes, northern Croats are of the same Slavic group plus some other people such as small number of Iranic that migrated with the Slavs and a small number of autochthonous of Pannonian plain. That happened during Slav-Avar expansion into Pannonia, east Alps, northern Croatia.
Belarusians don’t score Finnish ancestry in general. It’s not uprising as there were no known Finno-Ugric settlement on territories of present day Belarus. If you draw a line between Riga and Moscow — north-eastern region from this line was Finno-Ugric, while south-western of the line was ancient Balto-Slavic settlement.
Western Ukrainians score more Balkan and other south European admixture. It could be the influence of ancestors of Dacians, Vlachs, Iranic that roamed steppes of Ukraine. I am not sure.