The items showing in the Milestone Burnup Chart and the Cumulative Flow Charts include all work items that roll up to the Milestone.
The count shown by the Scheduled tab icon represents only work items directly associated to the Milestone.
The list of items displayed in the Scheduled tab will show any item that is directly associated to the Milestone and its children, with no regard to whether the children are directly associated.
Please see the Help article for more: Create and Manage Milestones
The below video on this difference, which should add some clarity: