On top of the reasons Meredith Hoffman gives for spelling variations, you can add 21st Century transcriptions of 19th Century penmanship.  My husband has Serings and Serrings that are often interpreted as Lering and Lerring.  If you go to the page of the census or marriage and actually see the document you can tell the difference.  The initial loop of the S starts on the bottom line while the L starts halfway up, otherwise the letters are identical.  You have to search both letter versions....then look at the document.