It really depends on the quality of the graphite in the ore...

It really depends on the quality of the graphite in the ore body, and that depends on the crystalline structure when it formed.
Many deposits will have varying consistency of crystalline structure. The more crystalline, and in the right way will not only mean more high quality graphene ( few layer), but from what I understand it’s actually easier to liberate. It’s like a splitting a lovely ordered stack of cards, rather than a jumble of different thicknesses and shapes.
I think there’s a couple of million of graphene ( VFL) layers in a few millimeters of graphite, so to have a lovely ordered stack is rareish. And of course the cost to liberate then is much cheaper then.