Fionn, the story I've heard is that Queen Elizabeth I intended to make John(?) Cavendish Duke of Derbyshire, but the scribe who wrote out the actual Letters Patent misread the scribbled instructions and wrote Devonshire.

As the Queen had applied her seal to the document before the mistake was noticed, he had to be Devonshire henceforth, as she wouldn't make him Derbyshire as well!

(And if I remember correctly he actually came from Suffolk, and rose as a protege of Cardinal Wolsey)

WassaiL! V