The RPM has to be constant (obvious). Therefore the linear speed of the disk surface is going to be a smooth function of radius.
The electronics will have several modes (not an infinitely smooth adjustment), and they'll have to 'switch gears' at certain points.
Therefore the areal density has to be variable across the surface.
The only way it could be held constant would be if the electronics could smoothly adjust to match the physics. And that's obviously not practical when working at the bleeding edge of what's technically possible. It would be practical to be smooth and continuous with low speed garden variety electronics, but not when using the sort of hardware ASICS and gate arrays that would be necessary at these data speeds.
This is all perfectly obvious. Why would anyone assume different?
One can focus on the peak areal density and then mention the typical special efficiency that can be achieved due the limitations of technology.