The Statement/FlexReport total will show to be higher than Cost History report.
The difference can be due to how Cost History is grouped by 'UsageStartDate' and Statements/FlexReports look at 'BillingPeriodStartDate'.
Services like "Amazon Registrar", "CapacityBlockFee" and "OCBAWS reInvent Passes" are billed on a certain month but their UsageStartDate is in the future.
Example:
Cost History will show the "OCBAWS reInvent Passes" charge in November 2024 because that is when the usage date started.
Statements/FlexReports will show the charge in October 2024 because that is when the customer was billed for this service.
To confirm this: