If you click a site that highlights lots of polygons you may need to use the site size filter to highlight smaller sites.
Sometimes sites are owned by a company that has mining/mineral rights below the surface. The reason you are able to select a few of them within this is due to the fact that some parts of land may have been sold to other companies. If you were to buy a title register for one of the available site you are able to select it would show the relation the site has to the mineral rights. If you have an interest in a site that you're unable to select you would ultimately need to get in contact with the owner of the mineral rights land. However, if you wanted to scope out smaller sites then you are able to use the max site size filter and adjust it to a smaller site.
There can be other circumstances for multiple sites showing with other sites. You can simply purchase the title document for the site to understand more.