The thumbails could be live, if the application developers chose to allow it. The stock music player still updates it's UI when mimimized, but it only does so every 10 seconds rather than at the same rate as when maximized. This saves quite a few CPU cycles, since it's likely that the music player will spend a lot of time in mimimized state. I imagine the clock application behaves in a similar way, with the hour/minute hands still being updated, but not the second hand.