It's common for hard drive manufacturer's to misrepresent how much storage space hard disks have. This misrepresentation is easy because the general public doesn't know what a gigabyte really is.

Here's the lowdown for you non-geeks: Computers count in binary, 1's and 0's instead of 1-10 like humans. So computers represent numbers in base 2 instead of base 10. Note: I'll use 2^3 to mean 2 to the exponent 3.

An interesting thing about base 2 is this:
2^10 bytes = 1024, or about 1,000 (kilobyte)
2^20 bytes = 1024x1024, or about 1,000,000 (megabyte)
3^30 bytes = 1024x1024x1024, or about 1,000,000,000 (gigabyte)

That about is where the misrepresentation lies. It's not uncommon to find that an "80 gigabyte" hard drive only has about 80,000,000,000 million bytes. OK, what's the problem with that?

Well 80 gigabytes is actually 80x1024x1024x1024 = 85,899,345,920 bytes. That's a difference of 5,899,345,920 bytes! or about 5.5 (real) gigabytes! That's a lot of "missing" storage space.

So how did this happen? Well, geeks don't really care that much because we know about it. It's been going on for years. The problem is that once hard drives started getting into the gigabyte range, the error (using 1000 instead of 1024) kept getting worse and the number was off by more and more. Then the average joe six-pack consumer started noticing too ...

I told a non-geek friend about this and it blew her away, the rest of us will probably just say "so what?". :) The good news for consumers is that there is legal action pending about this issue (the link escapes me at the moment). As computers enter the mainstream more and more us engineers have to remember that regular people can't count in base 2 in their heads. :)