The Internal Endpoint used on the website delivers more fields than the endpoint in the API.
Internal Endpoint: https://go.xero.com/api/projects/projects?status=INPROGRESS

API Endpoint: https://api.xero.com/projects.xro/2.0/projects?status=INPROGRESS

While the internal Endpoint delivers the contactName, the external endpoint does only provide the contactId. To fetch the Contact name a user needs to get access to the whole accounting module which is nonsense. The contactName should also be delivered where the id is delivered.