Preprocess functions not always called in views_slideshow_ddblock module

I have an issue that preprocess functions are not called in all cases with the views_slideshow_ddblock module.

The issue: http://drupal.org/node/566766

A workaround is to remove the code from the preprocess function in the module and
add the code to the preprocess functions in the template.php file of the theme.

But, this does not make sense to me.

I thought I understand the theming system, but obviously not good enough. That is why I post this
in this more general place to get some help with this.

Can someone explain why preprocess functions are not called in all cases. After a change in the node, a change in the view. (reloading the page calls the preprocess function again)

Why does this works if the code of the preprocess functions in the module is moved to the theme layer preprocess functions.

Thanks

Philip Blaauw