Test Case:

Jquery:

      $('#block').animate({left: 20}, 500);
      $('#block').animate({left: 20}, 500);
      $('#block').animate({left: -20}, 500);

Html:

<div id="block" style="background: green; height: 100px; width: 100px; position: relative"/>

Result:

<div style="background: green; height: 100px; width: 100px; position: relative; left: 0px;" id="block">
</div>

It Appears that when animating any css property, using a positive then a negative value, the property animates perfectly to the positive value, but it's left at 0 after animating on a negative, no matter how much was take away.