When placing objects (vector, bitmap, etc) on a artboard, the position is relative to the artboard boundary. For example, if I have artboard 1 placed at position 0,0 and I place a rectangle at position 0,0, it will be placed in the upper left corner of the artboard 1. If I have artboard 2 placed at position 12,12 and I place a rectangle at position 0,0, it will be placed at the upper left corner of artboard 2.
When I nest artboards, the nested artboard position is relative to the entire document. In the example above, if I place a nested artboard in artboard 2, and place it at position 0,0, it will show up on top of artboard 1, not in the upper left corner of artboard 2 like the rectangle in the example.
It seems that when I nest an artboard into another artboard, the position reference should be the same as if it were an object (relative to the parent artboard boundary).