ISO 9241 defines user experience as:
a person's perceptions and responses that result from the use and/or anticipated use of a product, system or service.
Based on this definition, we would expect that usability froms part of a user's experience when they set out to achieve a goal by accomplishing tasks using a product or service.
In the same document, human-centred quality is defined as:
usability, accessibility, user experience and reduced risk
So the question I would like to pose is this: If the purpose of applying a human centred design is to improve the user experience of a product or service, why is the definition of user experience more inclusive (i.e. includes usability) than the definition of human-centred quality (where usability is another element alongside user experience)?