"Waiting for Action" is basically the same as "All Pending Merges", except it only shows the ones that are highlighted as fully cooked and ready to eat.
If you keep track of your proposals, it would be useful to have a version of this limited to Initiated by Me, but on the face of it that option isn't on the list, though you can get it by editing the URL (canAct=1&requested=1234567).
Going to Waiting for Action by Me only has the same effect as switching from All of WikiTree to My Watchlist.
What you've discovered is that if you then choose Pending Merges Initiated by Me, you get two things together, so what comes back is Initiated by Me + Waiting for Action + My Watchlist.
And if you then choose All of WikiTree, you've got to the desired invisible option, Initiated by Me + Waiting for Action + All of WikiTree.