I would say it is a configuration issue, but how to overcome it is the question. The problem is how does your ISP determine the download figure, is it at ISP or is there some monitoring program on your system ?

The avast web shield intercepts http traffic into its localhost (127.0.0.1) proxy so that it can scan content before it arrives ultimately at your browser cache so the monitoring process may be double counting.

Do you have to use a proxy to connect to your ISP (possibly to monitor your bandwidth) ?