Hey scott reed, my understanding is that those settings are specific to the page type. In my example it's per content instance of the page, meaning that you can have multiple pages of the same type that have different 'Available Page Types'. The content type selector that I described is available on every page.

The feature described in this post came from the idea of having more control over which pages can be created under a certain page — regardless of the page type that's being used.

I do see now that this might be somewhat confusing.