In Windws 10, why do folders in libraries or Quick Access not show all files in the actual folders?

In the Navigation tool, Windows 10 provides a list of ways to access folders and files, including Quick Access and a group of other file or folder names. If I click on something in the group of folder names, not all files in that folder are shown eg. 'Documents' is one folder name, but if I search under OS C: and find my 'User' name and the the relevant folder, there are more files there than show up using the group. This is frustrating as looking under 'OS C: Users' is time consuming and requires lots of clicks, and I'd have thought the main purpose of having groups listed was to avoid this.

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