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

[The Saxman]

I am struggling in understanding why this JSFiddle I set up with Isotope\[code\]<a href="http://jsfiddle.net/PpAdu/">fiddle</a>\[/code\]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? Thank you in advance!