What do the storage numbers mean?

[AllThingsPure]

I have a Samsung Galaxy S 8GB. Storage Settings shows:

USB Storage: Total space 5.78GB System storage: Available space 1.29GB



I have a Samsung Galaxy R, the box says 8GB. Settings Storage shows:

USB storage: Total space: 3.75GB Device memory Available space 1.73GB


I have a Samsung Note (16GB), Settings Storage shows:

USB storage: Total space 11.07GB. System storage: Available space 1.79GB


How are the numbers suppose to add up? Why is there no Total space for the second parameter in each case?

 

[chanchan05]

Its divided into a couple of drives. System storage serves as app storage, separate from USB storage which you can use as a media storage unit, like an external USB card. Then part of this storage is used up for system files which make it work. Its like how when you buy a PC, and they say its a 500gb hard drive, but you only actually get about 450gb free space.