Zen Grids Two Columns Issue

SassMeister-input-HTML.html

<ul>
  <li>Item&nbsp;1</li>
  <li>Item&nbsp;2</li>
  <li>Item&nbsp;3</li>
  <li>Item&nbsp;4</li>
  <li>Item&nbsp;5</li>
  <li>Item&nbsp;6</li>
</ul>

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// Zen Grids (v1.4)
// ----

@import "zen";

ul {
  $zen-column-count: 2;
  $zen-gutter-width: 0px;
  
  border: 0.5em solid purple;
  display: block;
  width: 70%;
  
  @include zen-grid-container;
  
  li:nth-child(odd) {
    @include zen-clear();
    @include zen-grid-item(1, 1);
  }
  
  li:nth-child(even) {
    @include zen-grid-item(1, 2);
    margin-left: 50%;
  }
}

SassMeister-output.css

ul {
  border: 0.5em solid purple;
  display: block;
  width: 70%;
}
ul:before, ul:after {
  content: "";
  display: table;
}
ul:after {
  clear: both;
}
ul li:nth-child(odd) {
  clear: left;
  float: left;
  width: 100%;
  margin-left: 0%;
  margin-right: -100%;
  padding-left: 10px;
  padding-right: 10px;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  -ms-box-sizing: border-box;
  box-sizing: border-box;
  word-wrap: break-word;
}
ul li:nth-child(even) {
  float: left;
  width: 100%;
  margin-left: 100%;
  margin-right: -200%;
  padding-left: 10px;
  padding-right: 10px;
  -moz-box-sizing: border-box;
  -webkit-box-sizing: border-box;
  -ms-box-sizing: border-box;
  box-sizing: border-box;
  word-wrap: break-word;
  margin-left: 50%;
}

SassMeister-rendered.html

<ul>
  <li>Item&nbsp;1</li>
  <li>Item&nbsp;2</li>
  <li>Item&nbsp;3</li>
  <li>Item&nbsp;4</li>
  <li>Item&nbsp;5</li>
  <li>Item&nbsp;6</li>
</ul>