I'm not sure why thats confusing... The water is made to go under to create the compartment. The compartment is in the return section not before it. It still doesn't effect the volume or water level in the return compartment.

i.e. If you have a bathtub full of water with a pump taking water from one end to the other it wouldn't make any difference to the water level or the volume if you put a wall in the middle (other than displacement, but thats another story).

I'm only using the above analogy to make it easier to understand, the same as I used the "image the compartment is 12 inches away".