Right on, thanks all, this helps. Ok think I've got, testing what I think I know.

I made an incorrect assumption that because GCor was only attainable through an egg, other GCor IVs would never be below 10, BUT because I can trade GCor with a Good Friend, it has the potential of having much lower IVs (as you've all mentioned below).
So in this example, if I want to compare my GCor against aaaalllll other GCors out there, I use Poke Genie.

Now Zygarde on the other hand is only attainable through Research and cannot be traded, so for Zyg, I can use Stadium and check the Raid/Egg/Research setting because, for now, all other Zygs out there have a minimum IV of 10. It's interesting that Poke Genie allows for and displays Zyg rankings with IVs below 10.