Missing free space
There may be discrepancies between what free space is shown in Finder, in About This Mac’s Storage tab, and in Terminal. The culprit is usually Time Machine’s local snapshots, which can fill as much as 80 per cent of your disk space; they also include files you’ve deleted, so it can seem like this space hasn’t been reclaimed.
About This Mac counts all this as used space in the System category, while Finder counts it as “purgeable” and therefore available. It should be freed up as needed. Clicking Time Machine’s menu bar icon and choosing Back Up Now ought to nudge macOS to jettison the local snapshots and flag the space as truly free.
Other space may be taken up by the Home folders of other users on your Mac, if there are any. Those may be invisible to you. Spotlight’s index files and other system items take up some space. Finally, in APFS, any other volumes (which may be created by macOS itself) share free space within the low-level APFS “container,” but this is hidden; About This Mac and Disk Utility show it as “other volumes.”
About This Mac’s storage charts aren’t so easy to interpret in High Sierra without some technical knowledge. See above for tips on deciphering them.