bootstrap col spacing

spacing is not consistent

HTML

<script src="//code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<link rel="stylesheet" href="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<!-- 
  Bootstrap docs: https://getbootstrap.com/docs
-->

<div class="container">
<br>
<h6>
This row/deck contains two elements. You can see that even with col-md-4 it spans more than 2/3 of the width.
</h6>
  <div class="flex-row">
    <div class="card-deck">
      <div class="card col-12 col-md-4">
      
      </div>
      <div class="card col-12 col-md-4">
      
      </div>
    </div>
  </div> <br><br>
  <h6>
This row/deck contains three elements. You can see that each card spaces 1/3 of the width as expected.
</h6>
  <div class="flex-row">
    <div class="card-deck">
      <div class="card col-12 col-md-4">
      
      </div>
      <div class="card col-12 col-md-4">
      
      </div>
            <div class="card col-12 col-md-4">
      
      </div>
    </div>
  </div> 
</div>

CSS

.flex-row {
  background: #f8f9fa;
  margin-top: 20px;
}

.card {
  border: solid 2px #6c757d;
  padding: 10px;
}