Is it possible that your immigrant ancestor’s last name was translated before being written in a document or a census record?
While I doubt if my Aunt Wilhelmina Senf is enumerated as Wilhelmina Mustard (I’ve looked), other last names may have been translated into the local language. Your Schneider ancestor may have been enumerated as a Tailor/Taylor. Your Blanc ancestor may have been enumerated with the last name White. And your Verde ancestor may have been listed as a Green.
So it may be perfectly normal for me to be sitting at the computer looking for Mustard instead of going into the kitchen cabinet <grin>.