Dear Friend ,
Your question is right , but you need to understand and learn defination of GB in term of market and your PC.
Your PC says 1GB = 1.073741824 Billion bytes where
Market Defines 1 GB = 1.00 Billion bytes.
So you bought 16GB from market = 16 * 1 bn bytes = 14.8 * 1.073741824 bn bytes = 14.8 GB on you PC…!!!
Very simple maths…!!!