I had a weird issue on y MacBook today: after being low on space for what felt like months, I bit the bullet and deleted 50% of stuff from hard disk. I freed up about 250GB of files, emptied the trash, and checked under About this Mac – Storage. Sure enough, I finally had space again.
But when I tried to install something, which instantly told me I only had 14GB available, and that’s just not enough. I thought, “pardon?” 
I double-checked on the command line with df -h, and the output corroborated the statement of the application I wanted to install: 98% disk space used. I thought perhaps Disk Utility would bring clarification, perhaps via a First Aid run. But sadly it only confirmed what the df command saw: my disk was nearly full, and whatever 250GB I had deleted were somehow still lurking around. How? Why? What was going on here?
Thankfully I found this StackExchange thread discussing the same issue. Apparently this weirdness is caused by local Time Machine backups that have not been relayed to an external disk. I do recall that my MacBook has been telling me that no backups could be made for some time. Either way, the magic command looks like this:
tmutil thinlocalsnapshots / 1000000000000 1
No administrator privileges are necessary. It takes a moment to run, but apparently this radically clears up local backups. I had this problem with macOS 10.13.6 High Sierra. Maybe this helps anyone with similar issues.
Thanks to Tetsujin for making this excellent suggestion.