Hi Cindy. I subsequently tracked down my Marvin Hunt. He was the son of a first cousin of my grandfather Joseph Lafayette Hunt. Marvin's mother had died, and his father was living with his brother in that 1930 census. I know of no other details or reasons as to why Marvin was in my grandfather's home and listed as adopted. At least I solved the mystery from my standpoint. It is a perfect example, just like yours, as to why we should question facts from as official a source as the federal government if we have knowledge of other facts concerning the matter. At least we can explain what we found in a footnote to what the census has wrong. I'm glad you also came to a resolution of your confusing records from the census.
I found another odd one just today. A marriage record where Ancestry interpreted the first name as Jesse. However, if you follow the document and the handwriting throughout the document, you know it is not Jesse. According to how the writer wrote his or her letters, the name is actually Jepe, or Jep with a flourish on the end of the p. I suppose the person transcribing it for Ancestry couldn't figure out what the name was and guessed that it was Jesse.