Speed of response on the avast forums is the norm and not the exception, unlike many other support forums ;D

It appears from your figures that your provider is correct in only recording true download traffic. That will be confirmed over the longer period you monitor it.

I think netstat is most likely the issue as that would record all network traffic local and remote (internet) connections into the web shield proxy then to your browser cache and possibly also include the listening ports for the Internet Mail scanner, so it would be double counting the email traffic into the internet mail proxy and from that into your inbox, etc.

I too doubt there is any increased traffic as a result of avast.

Welcome to the forums.