In the US, Germany was more socially acceptable than Russian, Jewish, Polish or other Slavic. My mother said she was thrilled to be marrying somebody who was German and Sephardic, because she was Russian and Polish. I also have ancestors who were supposed to have come from Germany but came from Russia. 

Some people might have lived in East Prussia or some other 'German' area which later became Russia (my mother's mother's family came to the US from an area that was once New East Prussia before Napoleon), but in my case, the family didn't live there at the time.

And anyway 100 cM is not a close match for Jews.

It might also be that clerks thought they were German, as one set of ggrandparents, who obviously didn't give the information, were, 'George and Annie' with the right surname, both German (he was born in the Netherlands, she in Nassau, which at least was in Germany when Germany formed). So some neighbor probably knew Mr & Mrs., and that they sounded German, and the clerk added the rest.

"So how do I interpret those DNA matches tied to German ancestry? How might I link them to ancestors in Galicia and Ukraine? Just to be clear, the DNA matches tied to German ancestry have typical strengths of around 100 cM."