I think the confusion is because geographically Finland belongs to Scandinavia, but linguistic it does not. Swedes, Norwegians and Danes speaks pretty much the same language, kind of like very strong accents. Finland on the other hand has an entirely different language. Iceland has similar language but does not belong geographically.
I think the correct usage is:
Scandinavia: Sweden, Denmark, Norway
Nordic countries: Sweden, Denmark, Norway, Finland, Iceland
Maybe there are some members from Finland or Iceland that could give their input on this?