iframe height .. 100%?

[Winterworks]

I have an iframe, but when I set it to height="100%", why does it do this?

http://trasion.com/forums.php

When I set it to like "1000", it goes 1000px high, but 100% doesn't work? (neither does "auto")

How can I make it fit exactly in the frame?

[SEOvB]

#iframeID {
width:500px;
height:400px;
}

<iframe id="iframeID" blah blah blah">