Why Pen Drive/ Hard Disk Space Shows less than Stated Capacity

You may often get confused about the storage capacity of your hard disk or flash drives. A 160 GB Hard Disk shows only 148 GB. A 30 GB hard disk partition shows only 29.2 GB usable space. A 8 GB pen drive shows at most 7.56 GB storage capacity. But why the stated capacity of the drive can't be usable? Read the full post. This is our today's discussion. Few days ago my 8 GB Apacer Brand Flash Drive gets damaged. Its capacity was 7.44 GB. I went to the vendor and submitted this to get a replacement (if possible). And also purchased a new one (Same Brand). But this time I'm really disappointed. Though its stated capacity is 8 GB, it shows only 7.2 GB usable space. 800 MB less capacity. 
As the price gets half the quality follows the it. Two years ago I purchased that pen drive costing $12. But 5 days ago it takes less than $7. Well I'm afraid. I'm anxious about the service life of the new pen drive. 
Let's back to the discussion. Our discussion is about the actual usable capacity vs. listed capacity. And why actual space is always less than stated capacity. 
There are 3 different reasons behind it: 

• Different operating systems treat your disk differently. Windows OS may not show the disk space like Linux, Mac or Unix. There are little discrepancies between the file formats and algorithms used by different operating systems. (It doesn't matter much)

• Another reason is that- A portion of memory space is reserved for system filesand data sector for better performance. This is the primary reason behind the less usable space than the mentioned space. 

• The last reason but not the least is the calculation method used by the disk manufacturing companies. They consider 1 KB = 1000 Bytes. But your OS may consider it as 1 KB = 1024 KB. Here you are also losing a considerable amount of disk space. 

Look at the calculation below: Virtual Storage              Disk Storage1 MB = 1024 KB                      1 MB = 1000 KB1 GB = 1024 MB                      1 GB = 1000 MB8 GB = 8192 MB                      8 GB = 8000 MB7.2 GB = 7372 MB                   7.2 GB = 7200 MBSee details here . . .
So, you are getting 172 MB less space because of using 1 KB = 1000 Bytes!
Now what do you think? Shouldn't the manufacturer mention the actual usable memory on the packet? :)