Hard drive lost space?

[dovaskarr]

I bought my PC around 2013 and my C: drive had 500GB (490GB real amount) and D: drive had 1TB (981GB real amount) and I know that is okay. Up to this day I have reainstalled windows and had a LOT of new games, files etc on those 2 drives. Last month I realized that my C: drive is on 326GB and D: drive is on 605GB of space OVERALL!!!!
Not used, not free but overall. For my C: drive I'm not sure exactly that had 500GB but for D: drive I know it had that amount.
Screenshot of the discs (there is also a sistem reserved disc that I ignored from the installation of my windows)-
http://prntscr.com/i26nsj

In my disc managment it says that everything is okay and it should look like that.
Screenshot- http://prntscr.com/i26pns

Is it possible that over time Hard discs just lose space from aging or is there some other problem, and the most important, was I scammed at the beginning for the discs?

 

[USAFRet]

From your Disk Management window, you have one single physical drive.
Your C and D are just partitions on Disk 0.

326GB + 605GB = 931GB. Exactly what it a 1TB HDD is.

 

[dovaskarr]

Yeap, stupid of me to realize that. Thanks man

 

[King_V]

The mathematical explanation is that marketing of the hard drive sizes uses the standard idea of things being powers of 10. So, 1 TB is 1,000,000,000,000 (one trillion) bytes.

The PC calculates based on powers of 2. So, a kilobyte is 1024 (that is, 2^10) bytes, a megabyte is 2^10 kilobytes (or 2^20 bytes), etc.

So, 1 trillion bytes is less than 2^40 bytes, hence why it looks like the drive's total capacity is smaller than stated on the label.

 

[RolandJS]

And, the OS housekeeping takes some available byte-space for itself.