The Challenge week 9. Update to heritage quest

I learned from my father that his grandfather's name was Alexander Asmussen.  I changed my search to August Asmussen  and found him in the 1920 census along with my grandmother, Christine, my aunt Marie (age 3), and my uncle Roy (age 8 months).  They all had the last name of Asmussen.  Why did they go through the hassle of  changing their names to Sanders?  My father and aunt Jean weren't born yet.  They were born as Sanders's and showed up in the 1940 census.
   I searched for the two missing uncles, Peter and Lawrence, under Asmussen and still nothing.  Did they change their name to something else?  Did they report back to Germany before the 1920 census?  Were they both dead before 1920?

When I have access to Ancestry Library next week I hope to find out more.  I really want to find out more about the oldest brother that stayed in Germany and was the personal guard of Kaiser Wilhelm.