Why is the used space in the Windows properties of the drive larger than the used space displayed in the software?
The Windows Explorer and the TreeSize drive list do show the space that is physically allocated on the drive while TreeSize shows the space that is occupied by all files under a certain path. Please make sure that you have the view option "Allocated Space" activated when you are interested in the physically allocated space.
Another possibility 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. If you want to get notified if a folder cannot be scanned, please open the options dialog (File > Options) and enable "Show error messages during scan" under the option page "Scan > General". Turning on the Option "Track advanced file system features" in the Options dialog may result in more accurate results, because it tracks e.g. hardlinks, but slows down scans. If a drive letter points to a sub-folder of a network drive, the allocated space (correctly) reported by TreeSize may also be much smaller than the physically allocated space on this drive reported by the Windows Explorer because possibly the whole drive is not accessible through the network.
Beyond the space that is needed for storing the files itself, additional space is used for storing management data like the File Allocation Table of the file system or the boot sector. It is not possible to free this space with TreeSize or any other tool. This is usually 0.5 - 2% of the occupied space.
Another possibility is that you are using a Software RAID - like Windows offers it - which spreads the data with redundancy over several disks. These disks will appear as one logical volume and the failure of a single disk will not cause any data loss. But for storing the redundant information additional space is needed, which cannot be used for user data.
A special characteristics of Offline Files can lead to wrong values for the allocated space of stub files. To avoid this, either ensure that the user which runs the scans has full read access to the scanned file system.
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. You can check the size of the reserved storage in the "Storage Settings".