The result of marketing pushing base 10 numbers on an archiecture that is base 2. Fundamentally is caused by the difference of 10³ (1000) vs 2¹⁰ (1024).
Actual storage size of what you will buy is Amount = initial size * (1000/1024)^n where n is the power of 10^n for the magnitude (e.g kilo = 3, mega = 6, giga = 9, tera = 12)