As said here : http://yuilibrary.com/trac-archive/tickets/2528774.html
I have to request all attributes individually with getComputedStyle()
paddingTop: drag.get('node').getComputedStyle('paddingTop'),
paddingBottom: drag.get('node').getComputedStyle('paddingBottom'),
paddingRight: drag.get('node').getComputedStyle('paddingRight'),
paddingLeft: drag.get('node').getComputedStyle('paddingLeft')