I've noticed the same thing. I really do get the advertised 3 hours on just the tablet portion when I'm reading papers (with battery saver on), but plug the "9 hour" keyboard in and I only get 6 hours total. It seems like it has to do with the connectivity of the two pieces.
Also, the fact that I still have 100% battery overnight in hibernate, but land at ~96% if it's shut down, tells me that the "shut down drain" is not a natural effect of the battery self-discharging.
(i7/16GB/dGPU)