Well, just to clarify for those who don't know, that's because hard drive manufacturers measure a gigabyte as a billion bytes, in decimal.
However, that's not how data is stored. In reality, it's actually 1024^3 bytes, or 1,073,741,824 bytes, since it's binary (powers of 2, in this case 2^30). If you divide 160,000,000,000 (the number of bytes in a 160GB hard drive) by the actual size of a binary gigabyte, that comes out to 149 and (very little) change.
This is basically so hard drive manufacturers can make the hard drive sound bigger than it actually is. Yay marketing!