Spelling changes that minor (Riffel versus Riffle) are probably just random chance. It might be a matter of personal preference or, perhaps more often, it depends on how a record keeper - a clerk, census-taker, or priest - writes the name.
I descend from this same line of Riffles (Jacob's sister Margaret married her cousin Barnabas Riffle), and I have a couple of Ohio marriage records where the name is spelled differently on the same record. William Riffel married Bridget Riffle in 1883 - their names are both spelled Riffle in part of the record, then are both spelled Riffel in the part of the record that they may have filled out. Their son Bernard married in 1911 and clearly spelled his name Riffil, giving his parent's names the spelling Riffil, but it appears as Riffle in the 1920 and 1930 census (for Bernard and his parents). But Bernard's name is still spelled Riffil on his 1965 death record.