I believe you are correct in your assumption of literacy. It would also be the literacy of the census takers. If you read those you will find several variations also. My variation, Schoville, comes from my great great grandmother, Mary Schoville, who added the "H" and made sure there was always an "E". Therefore the descendents of my Great Grandfather Warren William Schoville, son of Michael and Mary (Hull) Schoville, use this spelling with the exception of one grandson, Clyde W Schoville, who removed the "H" and changed his last name back to Scoville