Right click on your drive and look at the properties. Does it show total capacity of 931GB, or something less?
Files always take up more space that you would think because they are stored in clusters, and the last cluster is usually not full, but it cannot be used for another file. Also, the more files and folders you have the more space used for the file and folder information.
Your example does seem extreme, so please provide more details.
If you Right Click and select Properties on the drive, you will probably notice that it says 1,000,000,000,000 bytes or more next to the Capacity section (about halfway down). Your drive is technically a 1TB drive by Western Digital's standards because it is 1 trillion bytes. However, a true terabyte isn't such a perfectly rounded-off number. Technically, the way Windows recognizes it, a true terabyte is a drive with 1,099,511,627,776 bytes (or 1024^4). That basically means there are 1024KB in 1 MB, 1024MB in 1 GB, and 1024GB in 1 TB. Most HDD manufacturers go by the even 1000, therefore it shows less storage in Windows. As long as "Capacity" in the Hard Drive Properties is at least 1,000,000,000,000, then you're not technically getting ripped off and the drive is working as it should.