Disk Graph can only show files that are not protected by macOS and, as a result, report more free space than expected. macOS protects directories such as /Library, /System or ~/.Trash (the Bin).

Starting from macOS 10.14, you can grant access to these protected system files.

1. Go to System Settings >Privacy & Security > Full Disk Access.
2. Click on the + button.
3. When prompted, enter your system password.
4. Select Disk Graph and click Open.

Note: even when enabling Full Disk Access, Disk Graph doesn’t necessarily get access to every file. For example, macOS will still prevent access to other users’ home directories.