Why Xvnc and graphical session cannot co-exist?

According to TigerVNC, you cannot login a VNC session if the same user has already signed in a graphical session.

You will not be able to start a Tigervnc server for a user who is already logged into a graphical session.

I know it is caused by Xvnc, which creates a new X server. x0vncserver in comparison can connect to an existing X server. So when I have a login server in GNOME, I will see black screen when I connect to VNC. The same happens vice versa.

Would anyone what’s the reason behind that? If Xvnc can create a new X server, why it cannot have independent login?

Thank you!