Even when you buy hard drives, same thing. E.g., if you take a 500GB drive, completely brand new from a shop and plug it in, you will see that the free space is less than 500GB.
You're not getting anything less but it's because computers uses binary system to count
As above, your computer "counts" 1 Gb as 1 000 000 000 bytes instead of 1 024 000 000 bytes so on every kilobyte level you "lose 2.4%", on megabyte level you "lose" 4.8% while on gigabyte level you "lose" 7.2%