Aristocrats sometimes go by their titles rather than their surnames. For example, the current Earl of Snowdon was previously Viscount Linley until his father died and was/is known professionally as David Linley rather his actual surname of Armstrong-Jones.
The Duchess of Abercorn, friend of the late Duke of Edinburgh was known as Sacha Abercorn rather than the family surname of Hamilton.
The previous Duke of Bedford was for many years styled as Marquess of Tavistock and was known as Robin Tavistock rather than the family surname Russell.
The heir to the Dukedom of Norfolk is styled as Earl of Arundel and he uses the professional name Henry Arundel rather than the family surname Fitzalan-Howard.
There are loads of examples of the above although it isn't always the case. It seems to be a matter of choice.