There are a few possibilities. One common item is versioning, if you routinely edit large files like PDFs that can grow quickly. Download DaisyDisk or similar.
Okay, if DaisyDisk is showing that it's probably just a Spotlight glitch. The categories & bar graph are built from Spotlight indices. https://support.apple.com/en-us/HT201716
Had the same issue on a MBP 2016 with Sierra. Rebuilt Spotlight index manually. Did nothing. Clicked on ‘Manage’ under the Storage tab in ‘About this Mac’, and subsequently clicked on ‘Documents’ in that list. That did it for me.
I would suggested looking at ~/Library and seeing how large that is. Anything in there gets categorized as "System" and it can contain large things (like apps downloaded by Steam, and other things).
Had the same issue on a MBP 2016 with Sierra. Rebuilt Spotlight index manually. Did nothing. Clicked on ‘Manage’ under the Storage tab in ‘About this Mac’, and subsequently clicked on ‘Documents’ in that list. That did it for me.
I would suggested looking at ~/Library and seeing how large that is. Anything in there gets categorized as "System" and it can contain large things (like apps downloaded by Steam, and other things).
My Library contains 4.5Gb
[doublepost=1481425817][/doublepost]Also, its says iTunes has only 10gb when I have 800+ Gb of movies. It must be considering my movies as system files.