Not sure if this is a real bug, but when using small values for length attributes (-1px or 0px or 1px), there is no visual difference in the shadow position. I opened a screenshot in Photoshop but the RGB values are identical.
Created attachment 13450 [details]
shadow position not changed when using small position values
Added an example in the attachment.
I think this is a bug in Core Graphics. I've noticed it while trying to fix box shadow repaint.
See also bug 14736.
Fixed in <http://trac.webkit.org/projects/webkit/changeset/28714>.