Some people have claimed that the internal usage counts against it.. but it doesnt.
I have a Desktop, server PC, and laptop. Most of my downloads happen on my desktop. I then copy them to the server, and to a network hard drive. I have done up to 20g at one time. So desktop to server, and desktop to drive, would be 40g of extra traffic (ontop of the 20g download). that day should be 60g if it was that way? Nope. only the 20g download.
The usage is not even calculated at the MODEM end as far as i am aware.. its at the head end..
So internal traffic, should NEVER go anywhere near.
Only time i could see MAYBE something happening that way.. would be say a streaming device, which was miss configured to maybe look at the external IP rathe than internal.. and the pc to device stream went OUT and back in... but then you would have an UPLOAD and DOWNLOAD both high.