Until quite recently, historically speaking, there were no rules for spelling in English. People spelled names however they thought they should be spelled. Different opinions as to how a name should be spelled resulted in different spellings of the same name. Also, there were differnt accents even in different parts of the same country, and the same name was often pronounced in several different ways. That too led to different spellings of the same name. There's virtually no name, not even the most simple, that isn't found spelled in various different ways in earlier times.
The name Culwell was originally Cauldwell. It was changed to Caldwell, then Coldwell, then, Colwell, and finally Culwell. There were times too, and places, where the different spellings overlapped. The same man may be found on record with his name spelled in several different ways.
My own Culwells, found in Parker County, Texas in the 1800's, are descended from a Cauldwell who lived in North Carolina in the late 1700's. Between the two spellings, they were found with their surnames spelled in all the variants given above.
It's unlikely your Culwells were originally from Scandanavia unless they had a Scandvaian name that meant "cold well", and anglicized it, or a Scandanavian name prounounced to sound like "cold well". The name, Cauldwell, is exclusive to the British Isles.
Also, most people use a certain spelling of a particular name because it's the variant that became most commen for people of that name, in the area where they lived, and so became the accepted spelling.