Some of it depends on threat level as well. There was an airport "incident" during our last trip, and screening became much stricter on the return trip as a result. Like most things these days, I am sure that they have threat assessment levels and respond accordingly; it is a balance between safety and speed of screening.
The technology employed is quickly becoming much better now as well. Watch the screen of the x-ray station next time you go through and I think you will be impressed with the level of detail they can see, zooming up and down through the bag to see objects hidden under other objects. My DW offered to take out her plastic bag of liquids on the way down and the agent just laughed saying "not necessary - we can tell EXACTLY what's in there".