Akamai does not base billing on the Traffic Reports, but rather based on their Usage Reports. These may show discrepancies in the volume, below a high level explanation of why this may be happening:
Please note the Traffic Reports are content delivery metrics and are used to understand the volume andtraffic patterns for a Web property. Contract Usage Reports show the metrics used for billing purposes and are service and contract dependent. These reports show the variable part of monthly charges. This report was specifically designed to provide a preview of contract usage metrics throughout the month, including month-to-date 95/5 computations. Traffic Reports are not reported in 95/5 while Contract Usage Reports are. 95/5 is calculated by taking data for every 5 minute data point and lining it up from lowest to highest value. The top 5% of those values is dropped. Hence the difference between the invoice and the report.