This can happen when you are categorizing by a perspective and have a reallocation rule enabled.
Reallocation rules are applied after the Statement Detail drilldowns are generated as they are meant to accurately reflect billing. Items that are getting reallocated to perspective groups with a reallocation rule will still appear in the Statement Detail drilldown of its original group (most commonly Assets Not Allocated). However, the totals in the report and in the tabular chart below will accurately reflect the reallocation.