When editing CCK fields, the selector is built with the types where the field really exists. This behavior has the nice effect that you can quickly check if the field being edited is shared by simple inspection of the selector options, and it offers a quick method to switch from one type to another to compare the instance related settings of the field on different content types.
A note for themers: The output for content type selector is rendered in the help section of the page, using
hook_help(). So if you cannot see the content type selector, please make sure your page template prints the