It seems you haven't looked into the BILLION BYTE versus BINARY BILLION BYTE comparisons. It's about 7 to more percent difference or 100 BILLION BYTE drives show as 93GB in Windows. You still got the 100 BILLION BYTES of storage.
A common question and sad the vendors don't always answer it with the supplied documentation.
Ok so i have a general question about external hard drives. I know that when you get an internal one and check disk space, it is usually a little short (say you have a 40 gig drive and yet you only get 37 out of it, I think because the operating system takes some of the space) so what is the excuse for external drives having less space (say I have a 120 gig drive (apparently as advertised) and only get 112 gigs out of it?