In iOS 7, part of Apple’s focus was on new interface elements like depth and motion to help better guide you through and between apps. These ideals can sometimes present themselves in delightful little ways, such as the physics you can tinker with on the lock screen and Notification Center.
Example: on the lock screen, tap and drag the camera icon up a little, then let go; it drops and bounces a little. Now, drag it up much farther, then swipe it down like you want to bounce it—which is exactly what it does. Same with Notification Center: swipe down from the top just a little to trigger it, then let go—it has a slight bounce once it lands at the bottom of your display. Now swipe down a little to trigger it again, then swipe down as if your life depended on it, and Notification Center bounces accordingly.