There are two reasons. First, some browsers do not load images, so displaying a page with embedded graphics will not cause the image to be requested and transmitted. Second, popular graphic browsers, such as Netscape's Navigator and Microsoft's Internet Explorer, allow the user to disable the automatic display of images. Some users choose to disable image loading so that pages are displayed faster. Pages do not always completely load.
Popular browsers display information as it is received.
If a page is partially loaded when the user interrupts, the transmitting user's quota tally is only charged for the portions of the files transmitted. Finally, file requests that result in errors, e.g., request files that don't exist or can't be read, do not currently incurr the 500-byte overhead charge.