When you visit a website, that website will load content from a variety of sources -- including images, videos, ads, embeds, etc. At the router level, there's no accurate way to tell which site is the main one you visited, and which ones just provided some of the content to assemble the page.
But there's good news! Your Router Limits includes a Noise Filter (look for the checkbox under "Show Filters" on the History page -- it's checked by default). The Noise Filter is constantly learning and improving and hides as many "uninteresting" records as possible (i.e., sites that serve ads, images, etc.). It's not perfect, but it does help hide a lot of the noise. If you uncheck it, you'll see the difference!
Getting some "noise" is part of the tradeoff of having a filter at the router level that covers your entire home, rather than having to install software on every one of your devices -- which is not practical, especially for devices in which that is not possible (game systems, TVs, etc).