Save and Preview buttons stay in the middile of a form with Zen sub-theme

I am using drupal 6.19. I installed CCK. I created a node type via Content management->Add content type.

I created a Zen sub-theme according to its instruction.
http://drupal.org/project/zen

This new type has eight fields. In the form, the Save and Preview buttons always stay in the middle of of the page, leaving a few fields below them.

What is more puzzling to me is that I tried almost all of the contributed Zen sub themes
http://drupal.org/node/340837

They all have this issue.

However, if I choose the built-in Garland theme, the Save and Preview buttons always reside at the bottom of a form.

Wrong with Zen or sub-themes? Or a feature of Zen? Or did I miss something or do something incorrectly?

Thanks for any help!

All the best.