I also agree that the advertise space is misleading, at least for the Surface RT. For example, on a PC or laptop, you can take advantage of the full advertised storage space (where GB refers to 1,000,000,000 bytes.) Just run your OS from a CD (e.g. Live Linux) or USB flash drive and format the HDD blank to store your data on.
From what I'm aware of, the Microsoft Surface RT tablet can only run its own bundled operating system, which if the case means that the space taken up by the OS and bundled Apps can never be made available for the user to make use of. On the other hand, it would be interesting to see if someone does manage to port Linux over to the Surface RT. If it's possible to fully run the OS on a USB stick on the Surface RT, then it's possible to make full use of the advertised storage space.