The Total Milestone Count on the Plan > Timeboxes page differs from the Project count on the Milestone's detail page. For example, on the timeboxes page it displays 12, but on the detail page it displays 10. Why is there a difference.
The difference is caused by what projects the user has access to view and can also occur when the milestone is attached to any closed projects.
Your permissions level determines which milestones you can see. Subscription and Workspace administrators can see all milestones, regardless of the project. However, project administrators, editors and viewers are limited to viewing milestones that are associated with projects that they have permission to see. Additionally the Total Project Count includes closed projects and the Projects Count only counts open projects.
If a milestone is associated with multiple projects, and you do not have access to all of those projects, you are unable to edit the project field for that milestone.
You can find additional information in the help article below: