Here is what I think is going on.
There is a thread that only mods/admins have access to where we discuss internal MR issues. That thread is over 1,000 posts and had a new post about 20 minutes ago, so shows up in the Mega Thread section. Since you don't have access to that thread as a non-staff member, the forum marks the thread as "Private" for you. Since, as a mod, I do have access to the thread the same thread shows up for me in that section.
I suspect what will happen is as soon as someone makes a post in a public mega thread, that Private tag will go away and you will be able to see whatever public thread was posted to.