jQueryUI draggable helper ok in IE, wrong position in Chrome, Safari

jQueryUI draggable helper ok in IE, wrong position in Chrome, Safari

I am struggling in understanding why this JSFiddle I set up with Isotope


http://jsfiddle.net/PpAdu/

(basically I set all elements as draggable)

it is functioning right under IE while when dragging in Chrome or Safari the helper position retains the distance from the first element. I.e. if you drag whichever element in IE it will be ok, while in Webkit only the first top-left element is correctly dragged...the other show the helper at a distance equal to the distance from the first block.

Can you help me? Is it a browser bug or a jQueryUI problem? 

Thank you in advance!