When viewing Charts on a Milestone, the count of work items does not match the count shown in the Scheduled tab.
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Furthermore, the number of items listed in the Scheduled tab does not match the count shown as an icon on the Scheduled tab.
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Why is there such a discrepancy with the item counts under a Milestone?
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: