See the screenshot of this Recent Changes page on MediaWiki.org.
For an entry it says "Project:Sandbox and one page added to category".
How does a user know what that one page is?
If one is using the T9148 ("Watch edits which add or remove pages from a category") changes to track adds/removes from a category for whatever reason, they would likely in some cases have to do some investigative work to see what that other added page is.
This should be somehow displayed so a user can see all the pages added to a category.