Bootstrap 3 navigation bar right margin is not supposed to be -15px

In my web page I have a Bootstrap 3 navbar:

<body>

<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
  <div class="navbar-header">
    <a class="navbar-brand" href="#">Teachlab</a>
  </div>

  <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">

    <ul class="nav navbar-nav navbar-right">
      <li><%= link_to "Home", '#' %></li>
      <li><%= link_to "Help", '#' %></li>
      <li><%= link_to "Sign in", '#' %></li>
    </ul>
  </div>
</nav>

<div class="container">
  <%= yield %>
</div>

</body>

The problem is that navigation is to close to the edge of the screen. If I remove navbar-fixed-top from the navigation bar, it's working as I want.

Any hints?