Why does the total size displayed by TreeSize Free does not match the size of the drive?
One possibility why the total size displayed by Tree is that not all parts of the drive could be scanned due to access restrictions. Therefore it is highly recommended to run TreeSize as administrator.
Please make sure that you have the view option "Allocated Space" activated when you are interested in the physically allocated space.
Another possibilty has been introduced with Windows 10 version 1903, the system reserverd storage. This space occupies about 7 GB by default and is used by Windows exclusively. It is not visible to other applications and currently not represented with the scan.