Neat Gutter Problem

NeatGutter

Graphic Example => http://imgur.com/yeOpJ

Relevant CSS: 

div.shelves {                                                                               
  9   background-color:rgba(51,51,51,0.2);                                                      
 10   @include span-columns(12);
 11   margin: 20px 0 150px 0;
 12   -webkit-border-radius: 15px;
 13   -moz-border-radius: 15px;
 14   border-radius: 15px;
 15   -moz-box-shadow: inset 0 0 23px -5px #000;
 16   -webkit-box-shadow: inset 0 0 23px -5px #000;
 17   box-shadow: inset 0 0 23px -5px #000;
 18
 19 }
 20 
 21 div.cover_box {
 22   @include span-columns(3 of 12);
 23   @include nth-omega(4n);
 24   height:250px;
 25   text-align:center;
 26   margin: 10px 0 10px 0;
 27 }
 28 
 29 div.pub_logo {
 30   @include span-columns(12);
 31   margin: 10px 0 10px 0;
 32   text-align:center;
 33 }
 34 
 35 img.logo_img {
 36   max-height: 250px;
 37   margin: 0 auto;
 38   display: block;
 39 }
 40 
 41 .home_comic_cover {
 42   margin: 0 auto;
 43   width: 150px;
 44   display: block;
 45 }

(The only other CSS is in another file, and it contains only the @import for the outer-container.)