Information given by the Storage section of About This Mac was often notoriously unreliable in older versions of macOS. Although it has improved in the last year or two, it often doesn’t give particularly helpful categories, and lumps things together as Other or System Data.
Before you start an intensive housekeeping session to clear away all of your unwanted files, you can try starting up in Safe mode (with the Shift key held), leaving your Mac a couple of minutes, then restarting normally. This clears various caches and can reclaim several GB with little effort on your part.