Based on those screenshots, it looks like there's some other shadow-casting light which is already causing the placeable to "self-shadow", which is the shadow you find annoying. This is either coming from some other light in the scene, somewhat behind it, or from one of the environmental lights such as the "sun" or the "interior light". If this is the case, you could probably make the self-shadowing go away by switching the lighting scheme to "Interior, Torch-Lit Only", though you might not be happy with having to make that change.
The volumetric shadows in NWN are very realistic, way more realistic than most computer games which include shadows. But you can run into situations apparently like this where you get shadows going two different ways because of (presumably) two different light sources and one of them isn't being cancelled out as nicely as it would be in real life under similar lighting conditions.
As to why the shading changes when panned vertically and not horizontally, this is almost certainly due to the "Shadow Opacity" setting in the Environmental Options dialog for the environmental lighting preset you're using.
Those are just my guesses, anyway.
