Hi, why am I losing decimal in that function

function UpdateAmount()
{
var tmp;
var mnt;

myName = "myControlName";
mnt = parseFloat(document.myForm.myTotalAmount.value);
mnt = mnt + parseFloat(document.myForm.elements(myName).value);
document.myForm.myTotalAmount.value = mnt;
}

The total are calculated correctly but on the web page I lose decimal. Why?

Thanks.