How is meshing done by the PowerSI tool?

Unlike 3D FEM, in hybrid solver, the meshes in the canvas represent how the tool meshes the dielectric within a cavity, not how the tool meshes the metal. Due to that, the tool did not mesh everywhere on the metal but only where there were cavities formed.

The cavities are decided by the tool and are not allowed for a user to define. 

You can say that there is no cavity formed beneath (not above) the metal where there are no meshes generated. This means that in a particular metal layer (VCC/GND), if there is no cavity beneath the metal layer, there will be no mesh in the metal layer.

As a result, the BOTTOM layer will never have any mesh, as there is no layer beneath it, resulting in no chance of getting cavities.

TOP layer - mesh

BOTTOM layer - No mesh:

Team Simtech 

Cadence Design Systems