Early census returns were mainly filled in by census collectors. Many people couldn't write. Some may not have known how to spell their names. The census collector would write down what he thought he heard. Result, a wide variety of spellings of even locally common surnames. Imagine the task of trying to work out how to spell names he'd never come across before, spoken in an unfamiliar Irish or Scottish brogue, in a language which may not have been the speaker's native one. The information was then copied into Census Enumerator's Book; original census form was then destroyed. So, plenty of scope for errors & changes. My Scottish (or Irish) McLean ancestor who went to England as a boy or young man could read and write. He signed his name with a large M, a tiny c, then a gap before a big capital L. This is how my family have spelled the name for the 170 years since. His English wife, could not write. Clerks registering births of their children spelt the surname various ways. They shared a house with another literate Scot in 1841&51 so the name on the census was McLean. When lodging with a local family the name in the census book was Macklean, because that's how locals pronounced it. A son was Mack Lean in a later census when living with his English stepfather who couldn't write. Another McLean family in the same town pronounced the name McLane, so that was recorded on 1841 census. Again, husband could write but wife couldn't. He signed McLean when he married. Children's births & baptisms were recorded with various spellings, maybe depending on which parent was present and how familiar the scribe was with Scottish names. There were only 5 couples of that name in the town. It was a rapidly expanding industrial town. Clergy and Registry Office clerks would have been inundated with births, baptisms, deaths & burials, so didn't have time to worry about spelling.
