From: Eric Hellman <[log in to unmask]>
Date: Mon, 12 Oct 2015 11:43:12 -0400

Probably you have some big files that are getting served in chunks. A
reasonable approach would be to assign some fraction of a download to
each 206 request. The number to choose would depend on the
characteristics of your files- it's not like there's one calculation
that works in all situations.

From the AWStats docs:

HTTP Status Codes:

HTTP status codes are returned by web servers to indicate the status
of a request. Codes 200 and 304 are used to tell the browser the page
can be viewed. 206 codes indicate partial downloading of content and
is reported in the Downloads section. All other codes generates hits
and traffic 'not seen' by the visitor. For example a return code 301
or 302 will tell the browser to ask another page. The browser will do
another hit and should finaly receive the page with a return code 200
and 304. All codes that are 'unseen' traffic are isolated by AWStats
in the HTTP Status report chart, enabled by the directives
ShowHTTPErrorsStats. in config file. You can also change value for
'not error' hits (set by default to 200 and 304 with the
ValidHTTPcodes directive.

Hope this helps. It's been a while since I worked on log analysis, but
I may be working on it again.

Eric