120GB SSD-Disk: More space "used" than the sum of all files

When I open the info-window about my start volume (120GB SSD), it says that 108GB are used, 12GB available. But the sum of the sizes of all folders and files (hidden objects included) in the root directory / is only around 90GB. What are these 18GB which are "used", but not locatable? Moreover, when I open the menu Apple > About This Mac, and click Storage, it says that 60GB is used for "Other Users" (I have one further user account on my computer), but Finder says that the whole /Users directory only consumes 57GB. The same window says that "System" consumes 27GB, but the sum of all files and folder under the root directory besides /Users and /Applications (that is /Library, /System, /privater, /user, /bin etc.) is only 21GB. So the main issue is that there are 18GB of storage space not available to me, but I cannot locate where this space is used up. I have already tried to restart the computer in safe mode, and to delete NVRAM and PRAM, but nothing changed. Where can I find these ominous 18GB? And is 27GB for "System" realistic? (My system: macOS 10.14.3 Mojave, MacBook Pro, SSD 120GB APFS)