Finellach said:CK system is also seriously flawed. The cultures are supposed to represent culture but in CK timeframe we can hardly speak about cultures but rather about (proto-)nations in a modern sense. For that reason we have ridiculous situations where Scandianvians are divided among modern ethnic lines while South and East Slavs are all under one tag. The main problem in this is that CK doesn't follows it's own established parametars. I am quite sure there were much greater differences in naming f.e. between Croats and Bulgarians than it was between Irish and Scots, yet the first are presented in one tag and the others as two separate tags while it should be the other way around.
I think the hand-waving and lack of definition with the Slavs has as much to do with avoiding potential flame-wars and controversies than anything else - the borders and even the existence clear of ethnic divisions between Bulgarians, Serbs, Croats and Bosnians in this period is still a hotly contested issue and often spills into modern politics, in a way that the boundaries between the Scandinavian peoples doesn't.
Putting them into a generic "South Slavic" culture avoids a million "WHY IS PROVINCE #1235 BULGARIAN WHEN IT SHOULD BE SERBIAN?" threads, replete with 19th century ethno-linguistic-racial maps and associated axe-grinding.