You are noticing a couple of issues.
First the drive manufacturers don't measure a megabyte and gigabyte as computer professionals would. So you loose many many bytes to this funny manufacturer math. Arguably fine for consumers until they use a tool that tells them otherwise that was written by a computer professional.
To a computer professional, a megabyte is;
1024*1024*1024 = 1,073,741,824= 1GB.
To the drive manufacturers a megabyte is;
1000*1000*1000 = 1,000,000,000 = 1GB.
So, if I did the math right, you loose 11,798,691,840 (11.7GB) of raw space on the 160GB drive to consumer math. The I.T. pro would expect to get the 11GB.
For my 160GB Seagate drive, Disk Utilitiy has this line;
Total Capacity : 149.1 GB (160,041,885,696 Bytes)
Where is that extra 41,885,696 bytes coming from? The manufacturer was nice enough to give me some bonus bytes. By the way, Get Info gives me the same answer you have. Perhaps that is part of issue two...
Second when a drive is formatted, an amount of the total is used in the formatting structure. This is too difficult to explain, perhaps wikipedia has a fair explanation. Basically some bits are used up as markers between blocks of user data. Also some bits are used for error checking.
Anyway, you have what you paid for, like it or not. The math is explained somewhere on the box for the drive, when you buy them.