为什么代码运行时，left的值在不停的变化，求帮助
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>my运动</title>
<!-- <link href="运动.css" type="text/css" rel="stylesheet"/>
<script src="运动.js" type="text/javascript"></script> -->
<style type="text/css">
*{
margin:0;
padding:0;
font-family:"宋体";
}
#myblock
{
width:200px;
height:200px;
background:#99CC33;
position:relative;
left:-200px;
top:0;
}
#myblock span
{
width:50px;
height:50px;
position:absolute;
left:200px;
top:150px;
background:#9900FF;
}
</style>
<script type="text/javascript">
function mouseOver(L){
var timer=null;
clearInterval(timer);
var adiv=document.getElementById("myblock");
timer= setInterval(function(){
var v=0;
if(adiv.offsetLeft>L){
v=-10;
}
else{
v=10;
}
if(adiv.offsetLeft==L){
clearInterval(timer);
}
else{
adiv.style.left=adiv.offsetLeft+v+'px';
}
},30);
}
</script>
</head>
<body>
<div id="myblock" onmouseover="mouseOver(0)" onmouseout="mouseOver(-200)" >
<span >一个</span>
</div>
</body>
</html>
