One reason for changes of names from the names people were registered with is for distinction between members of families. Mother & daughter both Jean, the daughter may have been known as Jane to distinguish between the two. In my own family, Agnes is a common name yet a cousin, Agnes was know as Millie and only on official documents was she given her correct name. In her case, Millie, came from a middle name of Milligan and no one has any idea why, as she was the only Agnes in her family. There are no absolute rules and considering clerks only wrote what they heard phonetically, it's not surprising that there are so many names with different spellings.
