I don't see why. A lot of Western Poland used to be German, and it's not like there's one German cuisine either. You don't get many Bavarians eating pickled herring with beets, but's it's classic cuisine in Berlin.
Also, you'd be surprised at how widespread pickled herring is in Bavaria. Herring has been a trading good for millennia in Europe, was and is still consumed in the winter months in Bavaria. You can easily get a "Fischsemmel" at the Oktoberfest in Munich. Bavarians also used to pickle everything for the winter: cucumbers, beets, cabbage, beans.
> Directly at the German-Polish border, you'll enter Eastern Europe, then when you arrive in the mentioned cities, you're suddenly back in Central Europe.
I mean I get that impression even before crossing the border to Poland. It's just very rural and not exactly wealthy on either side of the border.
The cuisine up here in Berlin/Brandenburg is much closer to that of Poland than it is to that of Bavaria. I say that as a person who lives in Berlin with Polish roommate, and travels pretty regularly to visit friends in Munich.