I've asked the same question myself!
Some of our family names evolved a lot also. In the days before computers, I think consistency wasn't as important, overall. Other times, illiteracy was a problem, and I think those who recorded the names were spelling phonetically, maybe some were too polite or too busy to ask for clarification of what they heard, or they thought they knew better.
In my family it happened most with Scottish and Irish names. Giffin becomes Givan in some records. My Scottish ancestor Dechman became then Diteman and Dykeman, possibly because the person wanted their name to be said a certain way, or because a clerk used the spelling they assumed or thought was right.
Other times it could be that the ancestor wanted to make the change, it still happens.