I got the same outcome. It seems like the number of vertices on the surface matter, and not the TrisRendered(). Still, it varies depending on the 'num' value. It varies between 48k and 64. Strange.
It seems to be discussed before:
http://www.blitzbasic.com/Community/posts.php?topic=48359
http://www.blitzbasic.com/Community/posts.php?topic=47912
It seems to be discussed before:
http://www.blitzbasic.com/Community/posts.php?topic=48359
http://www.blitzbasic.com/Community/posts.php?topic=47912