The aforelinked reminded me just how off the Notification Center feels in regards to its “depth” within iOS and it’s interaction with the Home screen. I agree with the comment “Shrike” makes in this linked-to post on the AppleInsider forums.
In short, the Multitasking Tray is “under” the home screen. When you activate it, the Home screen moves up to make way. The same happens with Home screen folders. Again, the icons in a folder are “under” the Home screen and when you open a folder the Home screen slides to accommodate.
However, when you slide the Notification Center down, it slides right on top of the Home screen, yet it uses the same “underneath” design (linen) that folders and the Multitasking Tray use.
Just try going back and forth a few times between the Multitasking Tray and the Notification Center and you’ll see what I mean.