The Bandwidth tool generates an overall summary of the amount of bandwidth that your site is using. This is generated from HTTP traffic (looking at your web site, downloading files, etc.) and possibly FTP traffic. However, this does not include any other type of traffic, the most notable of which is POP3 - e-mail. Depending on what files you send and receive through e-mail, this amount could be considerable. Contact your hosting administrator if you want to check how much bandwidth you are using.
To check how much bandwidth is being used:
Click on the Bandwidth link in the Web/FTP Stats area.
Browse the results.