There were some Germans, no doubt but you are trying to impose your false view that most of the Poles serving in German army were of German origin or were volunteers.
There were no Poles in the Wehrmacht, period! Get that into your head!
Again, the rules were the same to any other modern army of that time...18, male, healthy and citizen of the state...
Any Germans living in what became Poland after the Treaty of Versailles (but not only in Poland. Volksdeutsche lived in many of the new founded countries after WWI)
were asked if they would subscribe to the Volksliste, if not they decided to be Poles, if they subscribed under "I" they counted themselves as full blooded Germans, eligible for conscription.
That's all there is, really!
When in fact most of them were conscript who were forced to be a part of German army and more often then not their family were hostage.
That's just crap!
Ethnic Germans, which were active in the cause FOR Germany, in the inter-war years (1918 - 1939)
Persons, who declared themselves for the Germanic "Volkstum"
Persons, in occupied countries, who were certified of Germanic origin AND / OR "Eindeutschungsfähigkeit"
Polonisierte (polished) Germans, later racial worthy (active) enemies of the III. Reich; (fighting for Germany to gain the "Wehrwürde")
One estimate was that there were well over 500k. This included those that came from Galicia, Volhynia, the Baltic areas and Polish Germans who were resettled in the fall of 1939.
The following group accounted for about 200k people, Besserabia, Northern Bukovina, in mid 1940 and Southern Bukovina and the Dobruja in the fall of 1940.