Keywords in Sub-Pane vs. Project Keywords Window

In exploring how to make use of keywords I noticed that when I add a new keyword in the Keywords Sub-Pane I do not have the option of using a hierarchy but the keywords I add there are listed when I select the Project Keywords menu action I added to my Main Menu (and when I select “Show Project Keywords” from the menu in my Keywords Sub-Pane).

However, when I use the Project Keywords “window” to add keywords using a hierarchical system the keywords I add are not listed in the list found in my Keyword Sub-Pane. I can easily drag them from the Project Keywords window to the Keywords Sub-Pane obviously.

I am curious if there is a reason the keyword features/tools are set-up this way and am I missing a reason for the differences?