My search for an explanation of how Surface Snap works give me a shallow description here: https://help.sansar.com/hc/en-us/articles/115002098903-Working-with-objects-in-a-scene
You can use the Surface Snap feature to drag an object around a scene in such a way that it "snaps" to any surface it touches. To activate Surface Snap, click the Surface Snap button in the Transform menu at the top of the scene editor. Once Surface Snap is active, you can easily click and drag objects around your scene.
This has been discussed in the past, and I seem to recall that the answer came back that it works only when an object is first introduced into the scene. After that point surface snap has no influence over the precise positioning of an object.
Please note.... This is a request for OFFICIAL documentation from Linden Lab to be added to the Knowledge Base on how Surface Snap works. This is not a feature request.