I'm not sure if this is accurate or not, but if we trust the "Highlight updates" of the simulator on the different lockscreens it looks like the lockscreen is re-rendering the whole screen every frame. At most it would only need to be re-rendered every minute, and even then we could only update the part of the screen that contains the minute information (and maybe battery). Wouldn't doing that improve the idle battery life even further?
The lock screen in the Simulator is not handled in the same way as the device, which does only render once a minute. Think of the Simulator as simulating the lock behavior.