Because 'Documents' and ~/Documents are not the same thing.
The list on the left is categorizing your disk usage by type. You have 524GB of Applications on your disk, 26.71GB of documents, etc.
HOWEVER, just like those applications do not have to be stored in /Applications (Applications can be stored anywhere on disk, including in personal users' home directories), documents can be stored anywhere, too.
In addition, there is nothing here to say that the 26.71GB of 'document's relates to the current user.
Also, that 'document' label is a generic tag for any non-categorized file ('hmm.. it's not an application, or a photo, or a movie, so let's just call it a 'document').
so what's you're seeing here is 27GB of 'generic, uncategorized data files' that may be part of your account, or may be other user accounts on the system.
Your own ~/Documents folder is empty, which hints that you either store your documents elsewhere (iCloud, maybe? or in some other folder), but your ~/Documents folder is not the only place on disk where 'documents' reside.