New Post: SPServices.SPCascadeDropdowns - More than one seperate Parent/Child relationships on a form

You're seeing the difference between dropdowns with < 20 options and 20+ options. This is out of the box SharePoint behavior.

See my blog post: http://sympmarc.com/2010/05/19/two-types-of-sharepoint-forms-dropdowns/

You can convert the complex dropdown to a simple dropdown if you choose by setting simpleChild: true.

M.