Two competing explanations!
There are quite a few "surnames" which originated from placenames. Someone was granted a piece of land and took his name from that. If the pronunciation of the placename never changed OK, but if the name got modified a bit over the years, both surname and placename tended to keep the same pronunciation whereas the spelling remained unchanged (written down in legal documents etc) - I've seen this explanation given for "Fanshaw" also for Cholmodeley, Chumleigh; Mainwaring, Mannering; Marjoriebanks, Marshbanks. Similar position to Princess Di's family home at Althorp, pronounced locally as Alltrup.
Alternative view is that some families got a bit bored with their plain Anglo-Saxon names and decided to spice them upwith a "Frenchified" spelling. This did happen with some of the Smith/ Smythe families. All blacksmiths originally spelled their name Smythe (pronounced Smith). Lots changed the the new French spelling Smith in the Middle Ages. Much later some changed to the more "refined Smythe (pronounced Smythe) to be different from all the "common" Smiths. Some Smythes (pronounced Smith) who never got on the bandwagon at all find this all a bit funny.
You takes your money........