Poland Civ are different in campaigns. In 2nd of Barbarossa and 5th of Genghis Khan, Poland Civ are Slav.In the new campaigns, however, 5th of Ivaylo of The Last Khan, Poland Civ becomes Lithuanians.
The same country has 2 different civs, that is ridiculous. For me I think let Poland Civ be Lithuanians in all campaigns, for the Poland-Lithuanians Union, which is more historic.
The civilization assigned to a faction can vary between scenarios, depending on what the designers intended that faction to play.
This can also be done for the sake of variety, like in Barbarossa 4, where each of the 3 Italian cities cities is played by a different civ (Italians, Franks and Byzantines). Similarly, Attila 6 has 4 different civilizations for 4 roman enemies (Britons, Celts, Franks, Teutons).