When you visit a web page, your browser will generally store a copy of the page in an internal cache. If the server is set up correctly, when the page is visited again, your browser will tell the server the date of the cached version of the page, and if the page has not been updated since the last visit, the server responds with a "no update" message (I'm too lazy to look up the status code) and the browser will simply display the page from the cache rather than getting the page from the server. As the no-update message is very small, repeat traffic could be the cause.

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