Please take a look at the following illustration:
There are two vertical ducts coming from the mechanical equipment, yet only one of the ducts appears in plan view. Why is that?
Please download the model (2010 or 2011 version), and see if you can come up with the answer. Please use the comments section to provide your findings (or guess).
First person to post the correct answer wins! What's the prize? Your name will be in lights in Part 2 of this topic, where I'll expound upon the cause. (I disqualify myself, since I think I know the answer).