hard drives capacity is always smaller then advertised size cus of the formattings. formatting takes up a lot of space. bigger hard drive you loose more. i have a 13.6gb drive and formatted is about 12.7gb or so. right around 13gb. i also have a 40gb western digital and when i format it, it is around 38gb or little less. same for windows and mac format.
now what drive makers need to do is same as monitor. you can't say just 19" monitor. you must include viewable size as well. so 19" yet 18" viewable. hard drives need to be the same. so instead of just 40gb they need to say 40gb unformatted yet 38gb formatted, or whatever.