Xero API seems to return two different UUID's for the same user

We're using OAuth2 to allow users of our system to connect to Xero. Once the authorization succeeds, Xero provides an access token with information about the user who made the connection, including a xero_userid.

However, using this ID to find the user via the Users API fails with a 404. The Users API shows a different ID for the same user.

Is there a reason these are different? And how can we use the xero_userid returned in the Oauth2 flow to find to the Xero user via the Users API?

In XERO, the xero_user_id you extracted from access_token is the internal userID that XERO uses to recognize the contact. So If you use the same the GET API will return 404 as it is not accepting the internal user ID, instead of that If you pass the CONTACT ID of the User it will return 200 with details.

At the same time, If you want to get the login user details(contact information), better call the /User API directly with the token, it will return the details and the response contain the contactID of the user in UserID field (I know a bit complicated)

Please refer to the screenshot for more details.