Getting weird results when subtracting two identical numbers

Getting weird results when subtracting one number with the same number. I thought I was gonna get 0 as the result but thats not the case. Could anyone look at the code below and please tell me whats going on?

function Update ()

 {
 
     if (Input.GetMouseButtonDown(0))
     {
     oldMousePos = (Input.mousePosition.x/Screen.width);
     }

     if(Input.GetMouseButton(0))
     {
         print((Input.mousePosition.x/Screen.width) - oldMousePos);
     }
 }

If I write it like this instead I get the exact same numbers when just clicking. If I drag the mouse around holding the key down, only "oldMousePos" will keep the same ofcourse.

 print((Input.mousePosition.x/Screen.width) + "  " + oldMousePos);

Also posted it on the unity forums here: http://forum.unity3d.com/threads/111155-Getting-weird-results-when-subtracting-two-identical-numbers?p=737808#post737808