windows 8.1 - File size different between Explorer and Hard drives Properties

We have a MAC machine running on BootCamp for Windows 8.1 as well as the MAC. It's been runing for almost 1 year. We found that on the Windows side (C:\ - BOOTCAMP) that this machine is only 4GB free space out of 371 GB. So we are starting to troubleshoot to find out what causing this.

We checked on C:\ drive (including Hidden file/folders) and select all the files and right click PROPERTIES to see the size and we found is 306 GB. So we are missing 66 GB !!!

Any ideas what causing this?

BTW ... we are running Windows 8.1 and we backing up via File History to external network device. But we now the fact that if the machine is not connected to the network, it backup to local disk. Now, we did setup to cache offline 5% of the disk BUT could be this is the issue?

Thanks