I have never heard of the "droop slope" governor before, but will now add that to my list of considerations for the explanation! I will try to find out about it.
I'd like to educate you about the droop slope governor, but the truth is I don't understand it all that well myself (hydro-mechanical isochronous I understand quite well - and could probably build one from scratch with enough time and tools - droop slope not so much).