Why is the allocated space in the Windows properties of the drive larger than the space displayed by SpaceObServer?
The Windows Explorer shows the space that is physically allocated on the drive while SpaceObServer 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. 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. This is usually 0.5 - 2% of the occupied space. It is not possible to free this space with SpaceObServer or any other tool. 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.
On Thinkpad laptops the Rescue&Recovery feature may use a lot of disk space in the folder "RRBackup" which is not accessible to other applications.
If a drive letter points to a sub folder of a network drive, the allocated space (correctly) reported by SpaceObServer may also be much smaller than the physically allocated space on this drive reported by the Windows Explorer because it is possible the whole drive is not accessible through the network. A further possibility is that not all parts of the drive could be scanned due to access restrictions. Turning on the Option "Track NTFS Alternate Data Streams and NTFS Hardlinks" in the "Configure Scans" dialog may result to more accurate results but slightly slows down scans.