Quote - The sun can cause non-parallel shadows. Uneven double planed glass, wide angle camera lenses, curved walls...https://secure.flickr.com/photos/citadelmonkey/2750224736/in/pool-92891572@N00
https://secure.flickr.com/photos/lulatahula/2697745459/in/pool-92891572@N00

The effect is a bit severe in the render, though. I agree. But I'd have to re-render a similar set up to be convinced. Right now, I'm not inclined to bother.

In these photos, the shadows look like that because of perspective distortion and it looks fine because they match other parallel/perpendicular lines in the image (like the photo frames and window for example)

In your render OTOH the scene is almost isometric, while the shadows look like someone were standing outside shining a flashlight inside. It looks really odd.