Strange offset with Watir/Capybara and PhantomJS

I want to detect the location of elements on a page using Watir and PhantomJS. My second approach using Capybara resulted in the same offset.

While the elements on the left side look good, the right side is misaligned:

I made the screenshot before and after I grab the positions for each element with element.wd.location, but the offset is always the same. I used evaluate_script and .getBoundingClientRect() with Capybara.

One thing looks suspicious to me: The search input field is not loaded correctly and not only shows a misalignment, but also a different size and the magnifying glass isn't shown. I don't know if this causes the offset.

I tested it with pure PhantomJS 2.1.1 (phantomjs file.js):

var fs = require('fs');
var page = require('webpage').create();

page.viewportSize = {
  width: 1024,
  height: 768
};

page.open('http://en.wikipedia.org/', function() {
  var positions = page.evaluate(function() {
    positions = [];
    elements = document.getElementsByTagName('IMG');
    for (var i=0, l=elements.length; i<l; i++) {
      pos = elements[i].getBoundingClientRect();
      positions.push(pos.left + ' ' + pos.top);
    };
    return positions;
  });

  fs.write('test.txt', positions.join("\r\n"), 'w');

  page.render('test.png');
  phantom.exit();
});

Same result: If you open the test.png, you see the an image on the right (left: 952px, top: 259px), but the test.txt shows it shifted to the left (left: 891px).

Do you know what could cause this problem?