Xbox 360 Memory Question?

My Xbox 360 only says it has 451 MB total. I added up in the memory section of the dashboard all the memory i was using and all the memory that was free but it only adds up to 451. But i thought the smallest xbox was 20 G? It says when i open up the memory tab "memory unit" under storage devices. But i've seen on other xboxes that it says hard drive, not memory unit. But there is no memory unit plugged in. do i just have a different xbox360