Why there are two sleep modes? I am quite confused, I close the lid and then my mac book pro goes sleep and sometimes it seems to be in deep sleep and sometime it is not. I mean, sometimes when you open the lid, it will recover soon with a log-in window showing up quickly, sometimes it will show a white screen and a recovering progress bar at the bottom and then a few minutes later the log-in window.

What's the theory behind this? How can I prevent it from going to deep sleep mode? Thanks
The theory behind it is that it gets a lot more battery life when in deep sleep/hibernation than otherwise. Apple says after about an hour the computer should go into deep sleep.

See this question for further information and how to disable it. (The actual disabling instructions are in the question itself.)