Same calculation in drag and in drop gives different results

I'm using jqueryUI drag and drop to drag an object (ventil_roerHANDLE) on the X axis, then I have another object (ventil_roer) that is repositioned acording to the first objects x positon, and I do a calculation to determine how far on the Y axis the second object should move.

When I calculate in the drag function the calculation is slightly of, but when I do the same calculation in the drop function, it is correct - ant clues as to why?

I now have to reposition the second object again in the drop function to get it to where it should be, causing it to "jump" slightly when dropped

Link to project: Vasskrafta - interaktivt kraftverk

click on one of the two small grey houses, and then drag on the black valve with the green arrows

Here's the relevant code

sym.$('ventil_roerHANDLE').draggable( //MAGASINKRAFTVERK

{

  containment:container,

  axis: 'x',

  snap: 'draggableArea',

  snapmode: 'inner',

  snapTolerance: 10,

  drag: function( event, ui )

  {

       if(sym.MyGlobalVar_startPosition_mag == 0)

       {

            sym.MyGlobalVar_startPosition_mag = 1;

            sym.MyGlobalVar_yStartPosition = Math.round(sym.$('ventil_roerHANDLE').position().top);

            sym.MyGlobalVar_xStartPosition = Math.round(sym.$('ventil_roerHANDLE').position().left);

            sym.MyGlobalVar_yPrevPosition = Math.round(sym.$('ventil_roerHANDLE').position().top);

            sym.MyGlobalVar_xPrevPosition = Math.round(sym.$('ventil_roerHANDLE').position().left);

       }

       //utregninger for å posisjonere den synlige ventilen

       var xPosition = Math.round(sym.$('ventil_roerHANDLE').position().left);

       var xPositionCopy = Math.round(sym.$('ventil_roer').position().left);

       var yPositionCopy = Math.round(sym.$('ventil_roer').position().top);

       var xPosChange = xPositionCopy - xPosition;

       var yPosChange = xPosChange * 0.48;

       var newXPos = Math.round(sym.$('ventil_roerHANDLE').position().left);

       var newYPos = Math.round((sym.$('ventil_roer').position().top) - yPosChange);

       sym.MyGlobalVar_newYPos = newYPos;

       sym.MyGlobalVar_newXPos = newXPos;

       sym.$('ventil_roer').css(

       {

       'left': newXPos,

       'top': newYPos

       });

}

sym.$('draggableArea').droppable( //MAGASINKRAFTVERK

{

  tolerance: 'touch',

  drop: function()

  {

       //utregninger for å posisjonere den usynlige ventilen

       var xPosition = Math.round(sym.$('ventil_roerHANDLE').position().left);

       var xPositionCopy = sym.MyGlobalVar_xPrevPosition;

       var xPosChange = xPositionCopy - xPosition;

       var yPosChange = xPosChange * 0.48;

       var newXPos = Math.round(sym.$('ventil_roerHANDLE').position().left);

       var newYPos = Math.round((sym.$('ventil_roerHANDLE').position().top) - yPosChange);

       //bytt topp posisjon på den usynlige ventilen (HANDLE)

       sym.$('ventil_roerHANDLE').css(

       {

            'top': newYPos

       });

       //reset prev X og Y posisjoner

       sym.MyGlobalVar_xPrevPosition = Math.round(sym.$('ventil_roerHANDLE').position().left);

       sym.MyGlobalVar_yPrevPosition = Math.round(sym.$('ventil_roerHANDLE').position().top);

       //flytt  ventilen til HANDLEs posisjon

       sym.$('ventil_roer').css(

       {

            'left': sym.MyGlobalVar_xPrevPosition,

            'top': sym.MyGlobalVar_yPrevPosition

       });

}