Photoshop CS6 Quick Selection Tool: Always a part of the shadow is selected automatically

[BaCbDc]

I used the Quick Selection Tool for selection:

then I clicked "Refine Edge" and used Refine Radius Tool with View on black

and moved the mouse with left pressed button around the edges:

After clicking Ok I get an image with a wrong selection,

a part of the shadow has also been selected:

And that's always the same. After clicking Ok in "Refine Edge" I get always an image where

more than my selecection in the first step is marked. Always a part of the shadow is also  marked.

Why and how can I prevent this?

[Norman Sanders · Accepted solution]

Later versions of Refine Edge have improved. Consider an alternative:  the Blend if command.

If the brown object has a crisp edge place a blank layer below the image layer.

Double-click on the image layer in the Layers panel. It will bring up Layer Style. At the bottom is Blend If

If you want to retain the brown object slowly move the top left slider to the right (to about 80) until the object is silhouetted.

Choose Layer > Merge down.

[BaCbDc]

Thank you.