"RTK Drones still need GCPs, but only half as many"

I've heard this line repeated time and time again. I'm currently testing the accuracy of RTK only flights vs RTK + GCPs vs GCPs are varying density. 

I'm curious, does anyone know where this comes from and the logic behind it?

I'll be releasing my results in the next week from what I'm seeing, in the meantime I was hoping to hear other peoples observations.