If this computer has been upGraded several times over the years , this maybe the accumulated effect of all the upgrades.
OmniDiskSweeper Safe to use
GrandPerspective
How to delete Time Machine snapshots on your Mac
See used and available storage space on your Mac
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The final word from Apple on Managing the " Other/ System Data “ Category
Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.