Bootstrap grid wraps on less than 12 columns

In this jsfiddle, I am trying to get finer control by nesting columns

<div class='row'>
    <div class="col-md-6">
      <div class='row'>
      <div class="col-md-6">
        <div class='row'>
          <div class="col-md-1">1+</div>
          <div class="col-md-6">6+</div>
          <div class="col-md-5">5=12</div>
        </div>
      </div>
            <div class="col-md-6">
        <div class='row'>
          <div class="col-md-2">2+</div>
          <div class="col-md-6">6+</div>
          <div class="col-md-4">4=12</div>
        </div>
      </div>
      </div>
    </div>
    <div class="col-md-6">test</div>
</div>

However depending on the distribution, the columns will wrap, even though both add up to 12.

Why do the rows wrap on some (1/6/5) and not others (2/6/4)? And is there a limit to the number of columns I can nest?