Phantom js doesn't seem to handle em-based media queries properly when zoomed

test-phantom.js

var WebPage = require('webpage'),
    System = require('system');

var address = System.args[1];
var zoom = System.args[2];
var width = 700;

var page = WebPage.create();

page.viewportSize = {
  width: width*zoom,
  height: width*zoom
};
page.zoomFactor = zoom;

page.open(address);

page.onConsoleMessage = function(msg) { console.log(msg); };
page.onCallback = function(data) {
  if ( data === 'gtg' ) {
    page.close();
    phantom.exit(0);
  }
};
page.onLoadFinished = function(status) {
  page.evaluate(function() {
    function _phantomLoadCB() {
      if (document.readyState === 'complete') {
        var w = Math.max(document.documentElement.clientWidth, window.innerWidth || 0);
        var h = Math.max(document.documentElement.clientHeight, window.innerHeight || 0);

        console.log('window is '+[w,h].join('x')+' pixels');

        var div = document.createElement('div');
        div.style.width = "1em";
        document.body.appendChild(div);
        var pixels = div.offsetWidth;
        document.body.removeChild(div);
        console.log('1em becomes '+pixels+' pixels');

        var pixelTarget = w - 100;
        var emTarget = pixelTarget / pixels;

        if ( window.matchMedia('(min-width: '+emTarget+'em)').matches ) {
          console.log('window is at least '+emTarget+'em ('+(emTarget*pixels)+'px)');
        } else {
          console.log('window is NOT at least '+emTarget+'em ('+(emTarget*pixels)+'px)');
        }

        if ( window.matchMedia('(min-width: '+pixelTarget+'px)').matches ) {
          console.log('window is at least '+pixelTarget+'px');
        } else {
          console.log('window is NOT at least '+pixelTarget+'px');
        }

        setTimeout(function() { window.callPhantom('gtg'); }, 1000);
      }
    }

    document.onreadystatechange = _phantomLoadCB;
    _phantomLoadCB();
  });
};