windows 10 - Used space does not match sum of all files

I run Windows 10 in VirtualBox and get the following values regarding hard disk usage.

Used space: 12.2 GB.

Sum of all files: If I open Windows Explorer, select all files and choose properties, I get 17.8 GB on disk.

What's confusing is that used space is lower than the sum of all files. If anything, I would have expected it to be higher because the sum of all files depends on how Windows Explorer is configured regarding displaying system files. Plus all the NTFS stuff that is "invisible" in Explorer should show up in used space.

By the way, "Show hidden files, folders and drives" is selected and "Hide protected operating system files (Recommended)" is unchecked. If left at defaults, the sum of all files is about 1GB lower, which is OK and expected. But it's still way off compared to used space.

As a comparison, my Windows 7 host system has the following figures:

Used space: 97.1 GB
Sum of all files: 98.1GB (Size on disk: 98.6 GB)

With those numbers I can live. About 1% difference is nothing to worry about IMHO, but where do the almost 50% difference in Windows 10 come from?