javascript计算li的个数，结果为啥不是4

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<div>
<ul id="u1">
<li>1</li>
<li>1</li>
<li>1</li>
<li>1</li>
</ul>
</div>
<script>  
window.onload = function() {
   var u = document.getElementsByID("u1");
   var linum = u.getElementsByTagName("li").length;
   document.write(linum);
}
</script>
</body>
</html>