Iframe space problem

Hi,
I am using a iframe for showing the context menu. In Iframe I have a <table> with the structure like this
<iframe>
<html>
<head>..... </head>
<body>
<table id="contextmenu">..... </table>
</body>
</html>
</iframe>

at runtime, I am setting the iframe's height equal to table('contextmenu')'s offsetheight. Now the problem is, when I display this iframe on request of context menu, it displays some space under the table. If I enable the border of that iframe and table, table's height is less than iframe's height though we have set them equal.
What can be the reason.

Thanks.