I think Hambon's is the best explaination.
Even though eastern this, middle that, western this, far that are geographical terms, many times other things (religion, ethnicity etc) are used to define the term.
Just look at Europe :
http://www.traveldocs.com/images/europe_b.jpg
Do you know why is Slovenia (east of Italy , south of Austria) Eastern European country while Austria is central/western European country and Czech Republic (north of Austria) is also said to be an Eastern European country ?
It has nothing to do with geography but with the political background of Austria and Slovenia/Czech Republic post WW II.
Look at Finland and how more east it is than Slovenia and Czech Republic. Finland's of course a Northern European country but many times it's put together with the western European countries and if i remember well, some guys here made fun of Seven for saying that some things don't happen in western Europe and when these things happened in Finland, you guys said that's a proof that the same things that are happening in USA are happening in W.Europe.
I mean, Finland's neighbour is Russia and Finland is eastern than half of the east European countries. Why is it then more of a western then it is an eastern european country ?