macOS not showing the actual free space I'm in the process of installing Windows 10 on my Macbook Pro (macOS 10.13.4/17E202, 256 GB of storage, no partition), but Boot Camp says I need at least 40 GB of storage.

So I freed up some space by removing some files, and "About My Mac" now shows almost 100 GB of free space on my Mac:



However, the Disk Utility shows only 33 GB of free storage... 3 times less:



The trash is empty, and I restarted my computer twice. I also used Onyx utility to free up some more space, empty caches, etc. But there is still this huge difference.

I tried to find "hidden files", that could cause this difference, using the Disk Inventory X app, but it shows 100 GB of free storage...

Another strange thing that happened: after I restart the computer, the status bar at the end of the finder shows 33 GB of free storage. But after 2-3 minutes, this number changes to about 100 GB... As if some cached values were used while the Mac is updating the folder sizes.

I'm pretty sure I have 100 GB of free storage, because I removed dozens of GB of photos and apps.

What's wrong? I'm kind of lost!
You probably have some local snapshots on your drive, you can try to list the local snapshots with 

tmutil listlocalsnapshots /


and with 

tmutil thinlocalsnapshots / 9999999999 1


you can delete all local snapshots and release the storage space.

You can find more information here How to thin your local Time Machine Snapshots on macOS High Sierra