Xbox 360 Advertising 20GB Storage but gives 13GB?

Datafuse.net is reporting another blip on the radar for troubled XBox 360.  This time around it appears Microsoft is advertising 20GB of storage for saved games, but in reality, it adds up to 13GB.  In actuallity, this isn't a bad thing as the drive reserves the remaining space for backward support of older gaming titles.  

"So where did the other 6-7GB go to? Is Bill storing his secrets on your Xbox 360? I certainly hope so, but that's not quite what's filling it. The lost diskspace is used for backwards compatibility with the older Xbox 1 games. The reason why they are doing this is because they need to emulate the Nvidia GPU they used in the Xbox 1 while they now use ATI GPU's. Nvidia didn't license the transistion, so Microsoft needs to emulate all this. "