How to make a shadow with box-shadow property equal on booth sides

How to make a shadow with box-shadow property equal on booth sides

I am trying to make a simple (ribbon like) effect with box-shadow property
, i have made it on the right side but on the left side the effect is not
the same and i cant understand why! Does somebody have any idea? my code:
JSFIDDLE.
do i have to change something here maybe ?
box-shadow: 10px 10px 100px rgba(0, 0, 0, 10);
Thank you in advance and have a nice day!