index function in bind problem

Hi,

     For attachment demo. When I first click 'replace-instance' trigger, The 'repeat index 'xforms:input control display '0', but index of repeat control is '2'. Then click 'replace-instance' trigger again, The 'repeat index 'xforms:input control display '2' and index of repeat control is '2'.

     Why are they difference? Can I use index function in bind?