Quite funny, when I read the title before checking the link, I first thought exactly about Poland voting preferences. For anyone interested: the difference in voting exists, because the east part of Poland is less developed and more rural in general. I remember that it is so in part because of how the Russian occupation was mostly of “steal as much as we can” variety, especially compared to Germans. It is also one of…
The territory that today is western poland was part of Germany pre-1939. It wasn't an occupation, it was just that territory of that country. Afaik, most of what was eastern Poland pre-1939 and was occupied by the Russians, nowadays is no longer Polish territory. The boundaries migrated west.
[0]: https://commons.wikimedia.org/wiki/File:GUS_languages1931_Po...