Why 2 pre cat o2 sensors? and one in the cat?

This is an 2004 jetta BBW motor, 1 BANK, 2 pre cat o2 sensors, and one inside the cat. Why are there 2 pre cat o2 sensors? What is really the point of having the second one pre cat if there is just one bank? Is this just a cali emission thing? And im guessing the one inside the cat is just like the post o2 sensor? I know the optimal reading for a post cat o2 sensor is steady ~.45, but what if its constantly at .2 and lower, because i was looking at this one car with a p0420 car today and the 2 pre cat o2 sensors were oscillating nicely but the post one, or the one in this case inside the cat was oscillating slowly at .03 to .15. It looks like a bad post cat sensor because a lean cat makes no sense to me(unless there was a leak which there wasnt) when the pre o2 sensors are reading good and car performs as it should. Correct me if im wrong please.