Hi Jörg, here's a bit more explanation on why that happens.

If you load the attached 3DM file you can see it has a curve in it, and although the curve is itself planar it is rotated around and not aligned with any of the Top/Front/Right views:



If you run offset on this curve you'll see that it will offset ok and when you move the mouse around you'll see that your mouse point is tracking along the plane that curve is in.

The origin of that tracking plane is the centroid of the curve's control polygon, and that origin point is what gets used for grid snap too.

So basically in the Offset command you end up using a different grid just for the duration of that command than the grid that you see inside of the viewports. Hope that explains the mysterious grid snap behavior! But like I mentioned I should be able to improve this in a future release of v3 so that it will look for the case where the curves are actually parallel to the view directions and use the construction plane origin for that particular case so that it will then grid snap onto the same grid lines that you see in the view.

- Michael