120GB SSD-Disk: More space "used" than the sum of all files
Yes 27 GB is realistic for Mojave and there may be no missing data, just delays in calculating and transient data being purged and accounted for.
I pay for a tool DaisyDisk to get me a second opinion on space, but all your numbers make sense to me.

https://daisydiskapp.com/

Unless you can’t free space following the guidelines and see things stack in a stable or good direction, I might do two things.

https://support.apple.com/en-us/HT206996


Be sure Time Machine is backing up everything to a local attached disk.
Boot to save mode next weekend one time and then again a month later.

See if the usage leaves out is my advice. I don’t use Clean My Mac, but you could try that for free for 7 days and also rent hundreds of utilities for a month for $10 - https://setapp.com/apps/cleanmymac
for me, the paid version of Daisy Disk and Apple’s cleanup do the trick very well for me on many Macs at home and work.