Disk storage huge difference compare to components

Hi

I do not understand the disk storage use compare to its components storage used:

• • At the Machine level, the Finder shows a use of 380Gb for the internal disk Macintosh HD.
  • Macintosh HD shows components (Users, Application, Library and System) totalising 180Gb of use.

From past discussions, I understand that there are OS hidden items which can justify ± 20Gb (snapshots, VM, blocks allocated, item used, ...). OmniDiskSweeper justifies 20Gb indeed. But in this case, the difference is 200Gb.

Is this a difference between storage used vs allocated?

Is there an app or a tool that can analyse and report these 200Gb?

I use a late 2009 iMac 21.5" OSX 10.10.5 Yosemite which suits my needs but my concern is to determine wether the difference is ok or originated by faulty blocks which would need a disk replacement or a new machine.

Many thanks in advance