I'm wondering if this is more a question like: I bought a 500 GB drive and it says it only has 465 GB capacity, and only 440 GB Free Space after I load up Windows on it?
IF that is your question, it really is two questions and here we go.
The "500 GB" specification is given by a hard drive manufacturer who understand the term Gigabyte to mean exactly 1,000,000,000 bytes. But Windows re-defines that term to mean 1024 x 1024 x 1024 bytes; that is, is means 1,073,741,824 bytes when it uses the term "GB". Hence 500,000,000,000 bytes shows up in Windows as 465.66 of those fatter "GB" things. It actually is exactly the same space. They just confuse you by using two different measurement tools to do the counting, and then make it worse by using the same term "GB" for the different tools!
The other "missing"space of 20 to 30 GB is actually all the files it takes to install Windows on your machine - many of them hidden so you can't add up their sizes. The space is not missing, it's just in use and not available for other things.