In many situations, this is due to reallocation rules that have been configured.
Within the Cost History reports, you can click on a value in the table or category in the chart to view the detailed lines that make up the costs of that group. If you have reallocation rules enabled, you might find the total in the drilldown doesn’t match the number you saw in the chart/table.
Reallocation rules allow you to move indirect costs to perspective groups based on conditions you configure. These are applied after the drilldown is generated and are only applied to the report chart and table below. Cost that has been reallocated will show within the drilldown of the original group. While the totals in the report and in the table below will reflect the cost after the reallocation.
In the below example, the drilldown shows costs items for the "Shared infra -1000017" group. The chart at the bottom (which reflects the adjusted cost report) shows this group has $17.41, which is after the reallocation rule is applied, but the drilldown shows $1,005.90, which is before the reallocation rule is applied. Also, notice that the total of all groups matches what is in the drilldown showing that cost was reallocated.
For more information on reallocation rules, see: Cost Reallocation