I want the one with more GeeBees!

Horrible title aside :D (it was all I could think of at the time), could someone explain to me why devices advertised with a certain amount of storage, never actually have the amount of storage? I know the OS and whatnot takes up space, but why don't manufacturers just include a few extra GBs to compensate? For example, if you buy a device that's advertised as having 16GBs of storage, why don't they just include two or three extra GBs so the usable space is actually 16GB?

It's not a huge issue, it's just something I've always wondered.