Because ProcessWire is so configurable, content can come from many different places, such as:
The page tree
Language translation files
Site template files
3rd party services / APIs / Webpages
An external database/application
This is entirely dependent upon how that section of the website was built, and since all we can see here is a photo representation, we can only guide you through an overview of your options.
This isn't a situation unique to ProcessWire - other CMSs, such as WordPress, would also suffer similar issues, depending on how plugins were used, and how the plugin authors implemented their modules.
As for your specific question: "Is it possible that I am not seeing the whole tree?" That depends on your account's access level. It also depends on whether there are sections hidden within the Admin node in the tree.
Is there any additional information that you can provide in relation to this page, and the "following text" that you wish to edit?