Why the "D-shaped" throttle body gasket?

Does anybody know why the 3.4L 5VZFE engine has a D-shaped throttle body gasket when both the throttle body and intake manifold have matching, round bores? I've heard different speculation and theories, but none of them stands out or sounds valid to me?

Blindly assuming the O-Shaped gasket would provide better airflow, and therefore power I swapped mine out this weekend. Now that I'm thinking about it more I'm wondering if I might possibly see worse gas mileage as a result of the MAF sensor picking up more airflow and increasing the fuel injection rate even if without changing my driving style?

 

Not sure why they have the D shape. It might have something to do with the PCV port right behind the gasket.

URD has replacement O shaped gaskets if you want to install one:

http://www.urdusa.com/product_info.php?cPath=9_171&products_id=199000038

You will not see any decrease in MPG. Airflow is controlled by the throttle.

Gadget