I agree, this is not clear enough in Cubase.
The visual (drawn) grid is not the same as the (Quantize) grid. The Adapt to Zoom means, that the Quantize grid changes based on the zoom factor. So if you zoom in, you get tiny grid for Quantize. If you zoom out, you get 1 bar Quantize grid. This should be in sync with the visual grid (but some exceptions).
If the Adapt to Zoom is disabled, you get fix Quantize grid always (independent on the zoom factor). But the drawn grid is still dependent on the zoom factor.