Because every time you browse, search or watch content YouTube learns and its search and discovery system starts to learn what you like. Because of that, you are seeing the videos that you like in the order that YouTube thinks is most relevant to you.
When you have incognito on, it shows you how someone will see your content. Incognito helps you get the most unbiased information you can to help understand your true rankings and performance.
I'm going to mark this answered and close the thread to further replies.