Overhead for Web surfing is considerably higher than something like E-mail. As much as 50% of the bandwidth gets eaten up by overhead protocols used in web surfing. This is because each graphical item on a page, no matter how small, gets downloaded as a separate transfer requiring its own overhead. Smart web designers work hard to minimize this effect, but you shouldn't lose any sleep over it. It's just the price we pay for such a convenient and flexible tool. If you are losing sleep about your usage, you can transfer large files using ftp. ftp is very efficent.
By measuring bandwidth at the lowest level we force an apples-to-apples comparison that is fair to all concerned. With metered service, we don't have to pack a ton of customers onto a single connection because you are paying for what consume. Because of this, metered service encourages us to make sure that your connection is clear and uncrowded.
*Note that we do not record what you download, just how much.