Actually the size has to do with HDs using Gibibytes for advertising. For example, I have a 1TB hd with no operating system, only backup files, but windows 7 says its only about 931GB. This is because the hd is actually 1 Tebibyte rather than 1 Terabyte. This is because 1 Gibibyte, or 1,000,000,000 bytes, is equal to 931.3 Megabytes or 931,300,000 bytes.

I'm guessing that companies use this system because it makes their HDs seem like they have more space than they really do, all though I'm not to sure about that. An operating systems ARE factored into the space being used on an HD.