I have this same question and I would like to elaborate if that helps. Go to Programs to Hierarchy tab and you will see a Add child button. When selecting the child that you want to add, you will be able to see the categories of investments. Select IDEA from the list, and show all.
Compare the number of Ideas with what you see in the Home --> Ideas list page --> Show all list. It might be a bit less than the earlier one.
It does not depend on Idea status, that I just checked. Anyone knows anything on this?