When going to Site Review website, it is showing www.youtube.com website having two categories.
Sites can have up to 4 categories and YouTube is a video platform which falls under Audio/Video Clips category. Mixed Content category is used to indicate that YouTube hosts a wide variety of content including potentially adult/offensive content.
Release : 6.7.x and 7.x
Component : Content Filtering
Youtube having multiple categories is expected as per explanations above.