Most webpages have moved to a "responsive" framework for rendering pages to adapt to multiple screen sizes. This uses a client-side stylesheet that dynamically changes the layout based simply on what fits on the screen, where before, the server would have to conduct a browser detect and push the correct "mobile" website content. It's a lightweight and simple sloution but not perfect. You can demonstrate this for yourself in any browser by scaling the browser down and watching the layout change as different visual components scale and snap into a different layouts to accomodate the available space.
Unfortunately, advertising blocks don't always scale well with some responsive page templates, causing content to flow in unexpected ways, which is what I observed on the website you referenced. You'll definitely notice different results from different websites based on the templates they are using and even the advertising services they partner with. Some do not strictly enforce industry-standard advertising size recommendations, so when you get pushed an advertising block that is larger than the space allowed, it can overflow and cause the responsive layout to change the layout of the entire page to accomodate it. This causes a terrible user experience and can be a nightmare if different size ads are refreshed and the page reflows again and again into different layouts as you are trying to view the content.
