To complete the expllanation:
Grid has two meanings:
-custom layout grid: Grid is tool setting. But technology has some requirement on minimum grid resolution.
- Router grid: Grid here is hypothesis grids that the routing will be over them (As much as possible).
Pitch is related to DRC.
pitch=spacing+width.
Place and route tools may suppose a little larger value for the pitch to have better routing.
So the spacing of routing grids are the value of pitch.