Additive resource usage (e.g. traffic) was not reset after the end of the billing period.
Check the option "Measurable" of the resource rate in the service plan, it shall be set to "Yes" for the resource usage to be reset after the end of the billing period.
Set the option "Measurable" to "Yes" in the service plan resource rate. To propagate change to already provisioned subscriptions their synchronizations with the changed service plan is required (option "Update limits" is obligatory).