Why Pendrive Storage or Hard disk storage is smaller than advertised ?

We all use pendrives,memorycards to store data and even hard disks. But when we observe the size of that storage device,For 4GB pendrive,only 3.6 GB is usable,and for 8GB pendrive ,7.56 GB only usable. Here is the reason for that Question.. We know that  1KB=1024 Bytes 1MB=1024 KB 1GB=1024 MB But during Manufacturing process They calculate as 1KB=1000 Bytes 1MB=1000 KB 1GB=1000 MB For this reason the storage will be less.