Meu palpite é o sistema restaurar arquivos de sombra. O site do geek explica: link
Why Your Computer Shows the Wrong Amount of Free Space
You’ll probably notice something odd about the amount of free space your hard drive contains, if you look closely. If you right-click your C: drive in Windows, you’ll see a certain amount of space referred to as “Used Space” – in the screenshot below, the hard disk contains 279 GB of files.
However, if you select all the files on your C: drive (including hidden files and Windows system files), right-click them, and select Properties, you’ll notice something odd. The amount of space used by files doesn’t match up with the amount of used space on your hard drive. Here, we have 272 GB worth of files on our C: drive – but Windows is using 279 GB of space. That’s a difference of 7 GB or so – where did all those GBs go?
It turns out that certain types of files don’t appear in Windows Explorer. Files in Windows’ aptly named “shadow storage,” also known as “shadow copies,” don’t appear here. The shadow storage contains System Restore points and previous versions of files for the Previous Versions feature in Windows Explorer.
To view the exact amount of storage used by shadow files on every hard drive attached to your system, you can run the command below. You’ll need to run it as Administrator – to open a Command Prompt window as Administrator, search for Command Prompt in the Start menu, right-click the Command Prompt shortcut, and select Run as administrator.
Run the following command in the Command Prompt window:
vssadmin list shadowstorage
As we can see in the command below, about 9 GB of space is used in our hard drive by the Windows Shadow Copy Storage. The difference above looked more like 7 GB, but that can be explained by rounding.