Bandwidth usage is different in Website Statistics
The program that produces our 'Website Statistics' reports uses a different method of counting bandwidth which is more traditional but unfortunately less accurate. For example, partial downloads are counted as complete transfers, and streamed media (podcasts and video for example) will report their full size even if they are stopped halfway.
In your main account we use a more precise way of measuring bandwidth, ensuring you get full use of your bandwidth allotment.
Also keep in mind your overall bandwidth usage shown in your account includes ftp uploads and downloads, while the 'Website Statistics' is only website visitors.