One possible explanation is political or legal. I had a rebellious Irish ancestor who got in trouble with the Brits in the early 20th century, came to America, and changed his name from English-sounding to Irish. His children knew his real name but were sworn not to reveal it. A family argument might explain a name change too. But surprising Robert would change from a Polish name to German unless he was working somewhere where Germans predominated and Poles got hassled. Lots of possibilities... Diane