Talking out my ass, but I suspect the way iOS handles multitasking is less ram heavy, too. Seems like it just takes a screenshot of the last state a program was in then restores it to that point when you actually open the app back up. That, versus actually having he program run in the background.
It seems to work like in the previous versions though, it continues to run in the background until it requires more ram for another app, at that point it shuts something down.