At the beginning, in the Ashkenazi custom, there were slight variations between Minhag Ostreich and Minhag Reinus. Minhag Ostreich spread out to Poland, Hungary, Russia and Lithuania, and every place east to the Elbe river.
Minhag Ostreich got the name "Minhag Polin" since the printers wanted to sell their books of Piyyutim etc., so they wrote in the first page the places these Piyyutim were said, and Poland was mentioned first because it had the biggest population, and this way in peoples memory the name became "Minhag Polin".
Since there were two types of Machzorim, when the printers wanted to sell Minhag Reinus Machzorim they called them "Ashkenaz" since this was the Minhag used in Ashkenaz (Germany) Switzerland and France.
When the Chassidim started to use the Chassidic Nusach (which they first called Nusach Ha'ari, then Nusach Sepharad). At that time they started calling Nusach Polin etc. also Nusach Ashkenaz, since it wasn't Nusach Sepharad but was nusach Ostreich which was one of the Ashkenazi customs, therefore at the 19th century you can find in Poland that the non Chasidic Nusach is called "Nusach Ashkenaz".
Michael
FRBSH