When you use the orthographic camera, all the camera rays are parallel. 
This means that when the points below the y=-1.5 plane are rendered, the
camera is effectively below this plane as well.
When the pixel representing <0,0,0> is rendered, the effective camera
location is <0,0,-6.5>, when <20,-10,0> is rendered, the camera is at
<20,-10,-6.5> etc.
So you are looking _under_ the horizontal plane.

Cheers, PoD.

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