Chrome: jQuery element width at zoom levels other than 100%

Question

Originally I set up a small test to find out how jQuery's .width() treats the different box models. Interestingly it shows the width defined in CSS minus the padding-left and padding-right for border-box. Now when I wrote this small test I zoomed the page so my colleague who was standing behind my desk could read the code from afar.

To see what I'm asking please proceed as follows:

1. Open the codepen example in Chrome: http://codepen.io/anon/pen/WoYeVg (it has to be on codepen since I cannot reproduce the same problem here in the snippet)
2. Click the first container. => 160
3. Now wait 5 seconds until the text reverts to "click me"
4. Set page zoom to 175% by holding CTRL (Windows) or cmd (Mac) and pressing + 4 times.
5. Now click the first container again. => 160.00001525878906

Main Question: Why is it not exactly 160 any more at 175%?

Side Question: Why does jQuery treat both box models different in the way it does?

In Firefox I was unable to reproduce what seems to me like a decimal rounding problem/error. Anyone who knows exactly what is going on here?

Please check the following snippet:

$(document).ready(function() {
  var elem;
  $("div").on("click", function() {
    elem = $(this);
    elem.text(elem.width());
    setTimeout(function() {
      elem.text("click me");
    }, 5000);
  });
})

.a, 
.b {
  background-color: #a00;
  color: white;
  margin: 50px;
  padding: 20px;
  width: 160px;
}

.a {
  box-sizing: content-box;
}

.b {
  box-sizing: border-box;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<!-- http://codepen.io/anon/pen/WoYeVg -->
<div class="a">click me</div>
<div class="b">click me</div>