Why is User Id changing?

Question

There's an account which is constantly changing it's ID, is this a common issue? what could cause this?

it has changed from _layouts/15/userdisp.aspx?ID=571 to 578 then 586 then 591

Could it be a sync issue?

Answer 1

To understand why there is changes in user id you should know something detailed about User Information List,

User Information List: As per the Name SharePoint User Information List stores information about a user by having some metadata set up for the user. Some examples are User Picture, Email, DisplayName, LoginName etc. This User Information list will be a hidden list and each site collection will have only one User Information list.

When we grant any user permissions to a user, they are added automatically to the hidden User Information list, a new item will be created in the User Information List storing some information about the user.

As we know there will be an ID associated with each item in a list, similarly there will be an ID associated with the item added in the User Information list. That id is called as Site User ID, only using this Site User ID we used to fetch the details of a user from the Site Collection or from User Information List of that site collection, to be more precise it is like fetching the item details from a list using the ID of the item.

So why does the Site User ID gets changed?

As said earlier, each site collection will have only one User Information list and the items added in the User Information list of Site Collection A will not be same as the User Information list of Site Collection B asn so on. So there will be difference in ID of the list item and that is why you are getting different Site User ID for a same person entity in different site collections.