I'm not sure I even understand where you're getting those numbers from, but I can tell you that "storing" files on One Drive does NOT necessarily mean the files aren't on your computer as well... it just means they're ALSO stored on One Drive. This isn't the case all the time, but it is more often than not.
Also consider that "Documents" includes a lot of things, not just your typical office docs. Everything in your Downloads folder, every file you store on your computer (including the ones synced from your One Drive), every email attachment, SMS text, calendar items, contacts, voice memos, notes, etc... Everything. You can see exactly what the numbers included by clicking on the Documents column and drilling down
