Because your code is buggy, its probably trying to protect you from yourself

have a look at the source of your page

</div>
</td>
</tr>
</table>
<table border="0" cellspacing="0" cellpadding="0" id="table6" width="776">
<tr>
<td></div><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
You've got a doctype declatration in the middle of the page

Your define pages should only have the body code,

when you are putting code in for the define pages you should only be putting in the code that is between the body tags (don't include the body tags), zencart takes care of all the rest, you seem to have placed an entire webpage in your define page, this will stuff you site up and will cause issues for crawler and some browsers