Tim,
I have these often. As far as I know it has to do with the meshing in Rhino. Since RhinoCam uses the mesh to create the toolpath, it must break the path into straight segments that move across the surface of each polygon. In your example, the “ridges” occur when the curvature changes and the settings on your controller tell it to stop the machine’s motion then restart in the next slightly different direction. So this momentary stopand start creates vibration that causes a bit of a ridge. (I believe that these settings can be changed per machine). With a finer mesh this can be alleviated to some extent since the direction change isn’t as great.
You can test this by making a surface that gradually changes from gentle corrugations to larger ones. At a certain point the controller will decide that the change in direction is too great to do without slowing to a stop and re-accelerating.
Nick