A Microsoft file hosting and synchronization service.
I have the same issue. The Mac Storage Manager is calculating that I have more in my documents than my hard drive size. It seems to be that the manager is calculating the full folder size for OneDrive and SharePoint vs the "On Disk" size
But if you ignore the colored graph, my "available disk space" is accurate (see image below).
First image is pre-OneDrive and Sharepoint (168GB available of 250GB with Documents at 5.38GB)
After Installing SharePoint and OneDrive shows 166 GB Free of 250GB but documents has jumped to 331GB which isn't possible on a 250GB HardDrive. If you look at the info for OneDrive and SharePoint they show 154 + 170 GB which explains the 300+ but OnDisk is only 3.8MB plus 4.4MB. So the 331 GB they are showing in the calculator isn't in fact on my HD (everything is in the cloud and on demand files is turned on). The OnDisk number increases and decreases as I open files and "free up space" which is correct.