Direct sunlight will cause Hi Contrast shadows, An Overcast will soften those shadows. thus softer shadows. A Photo is not as flexable as the eye. Direct light with a high contrast shadow, when looked at through the eye looks at the overall scene will look contrasty also, but the human eye acts like f stops on a camera, when you focas in on (example of the shadows under the tank.)
 
This will still be high contrast, but the eye can detect a greater range of contrast than your typical photo. Because of this, The Rendered Tank renders just like a Photo would in Direct light. If you want to change this, your going to need to use another light source so you can soften the shadows. 

Thus a scene with the tank in nature will show as the example did with hard shadows. This is why photographers use refeclors on models, a trick to soften the would be unflatering hard shadows of the model. either that or shoot the shot in the morning or evening.