Disk Space Usage Inaccurate?

Recently I (and a friend) each noticed that on our separate desktops, the used space on our single C drive was showing usage space about double what it had in the past 12 months, and what we expected to see when we looked. I have an SSD and he has a HDD.

On mine I have a 250G Samsung SSD single partition C drive showing about 220G used and 30G unused. When I totaled the used space in all my Folders (I right clicked on each folder and selected Properties to check usage . . . including checking the hidden folders) I get a total of 90G used. Even if I add to that the fact that my empty Recycle Bin is set to accommodate up to 14G and my Page file up to 6G I'm still at only 110G potential maximum used space, not the 220G WIN7 shows I am using. WIN seems to be double reporting used space if my calculations are correct.

My folder usage is as follows: WIN 45G; Program Files 6G; Program Files (x86) 5G; Users 19G;Nvidia 2G; $WIN WS 5G; Program Data 7G; Miscellaneous smaller folders 1G; Empty Recycle Bin . . . up to 14G; Page File capability . . . up to 6G capability. All this space would only account for the approximate 110G and not the 220G indicated as used in My Computer.

Anyone have some insight?

EDIT:
I think I just found the answer. It appears that System Protection (System Restore) is utilizing 106G which would then make sense as far as the disk utilization numbers go. Wonder why this would not show in File Explorer as a hidden file to let the user know this?

Are the computers running auto-create SRPs? SRPs can take quite a byte! Also, some security and monitoring programs log and log and log and log... -- search different log buckets.

Have a read if this thread:

C: Drive Used/Free Space WAY Different Than Actual? - Windows 7 Help Forums

And this link from the same thread: Computing the size of a directory is more than just adding file sizes The Old New Thing

So you cannot add folder size to get an idea of used space.

"...I think I just found the answer. It appears that System Protection (System Restore) is utilizing 106G which would then make sense as far as the disk utilization numbers go. Wonder why this would not show in File Explorer as a hidden file to let the user know this?..." I can't remember where right now, however, you can go into a "grey panel" and reduce the allocated byte-space or percentage of HD available byte-space; and you can delete all SRPs except for the most recent one.

To get a better idea run these:

Wiztree - get portable: Antibody Software - WizTree finds the files and folders using the most disk space on your hard drive

WinDirStat Portable: PortableApps.com: Portable Software/USB - Browse /WinDirStat Portable at SourceForge.net