This is just a guess but one of the news sites may have more kilbytes than you are allowed and they charged more?? I don't know other than that - every time we visit a site, we download everything that is on that site = pictures, html or other pages, music files if they play, even videos. If you use IE and go to Tools (I am trying to recall since I use Foxfire) and then Internet Options, and then under where it reads Temporary Internet Files, click Settings and View Files, you will see where they all are. You can delete the ones you don't want - prob most of them. It just gives you an idea of what you have. If you do not delete them on a regular basis, they do to other parts of hard drive and you can accumulate gigabytes of programs, images and files on your system.
Other than that, I have no idea why your ISP charges you so much. I have a flat rate - download what I want.
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer