How do you know a page has loaded? A complex page with ads, AJAX, WebSockets may be constantly busy. Most social buttons, ads, etc. are now loaded by callbacks, that usually finish after the page has rendered.
Most of that data flow, barring user interaction is much more limited compared to the initial load of controls, iframes, images, etc... you can visibly see the drop off..
If you look at the network tab in chrome dev tools, you can see when the dom ready event fires, the window load event, and when it really feels the content was done loading. That final load time is when the data flow lulls out for a bit.
If you look at the network tab in chrome dev tools, you can see when the dom ready event fires, the window load event, and when it really feels the content was done loading. That final load time is when the data flow lulls out for a bit.