Yeah, lots of stuff.
It's site-dependent: some pages may contain frames or objects that "play" something and/or constantly updating data, perhaps.
Maybe not constantly, but some sites - I open one and the same page in Chrome, and the icon says "o'k, fully loaded", but my data-meter keeps showing high speeds for several tens of seconds or something. I guess it might be banners etc. - content created elsewhere and only hosted on the site, on its pages.
Well, you might try disabling/disallowing some dynamic content, depends on which settings the browser has for that.
:sherlock: