Javascript Division in IE10 compatibility mode

Can someone tell me why the following function works in Firefox, Chrome, and IE10 but NOT when running IE10 in compatibility mode?

The error in console is here:   gpm = (dia*dia*2.56*horz)/Math.sqrt(vert);

The error is SCRIPT438: Object doesn't support this property or method

<script type="text/javascript">
function calcGpm() {
 var dia = document.forms[0].dia.value;
 var horz = document.forms[0].horz.value;
 var vert = document.forms[0].vert.value;
 
 gpm = (dia*dia*2.56*horz)/Math.sqrt(vert);
 gpm2 = Math.round(gpm*Math.pow(10,2))/Math.pow(10,2);
 if(gpm2 > 0 ) {
       document.getElementById('gpm').innerHTML = '<b>'+gpm2+'</b>';
      document.getElementById('gph').innerHTML = commafyNumber(gpm2*60);
      document.getElementById('gpd').innerHTML = commafyNumber(gpm2*1440);
      }
 
}
function commafyNumber( number ) {
  number = '' + number
  var withcommas = number.replace(/(^|[^\w.])(\d{4,})/g, function($0, $1, $2) {
    return $1 + $2.replace(/\d(?=(?:\d\d\d)+(?!\d))/g, "$&,")
  })
  return withcommas
}
</script>