That's interesting. I didn't know that.

Why is this ( for my own edification I ask, not that I doubt that you are correct )?

I would normally expect that the squared off edges would cause mini vortexes at the edge, as the sharp "break" typically creates a low pressure area that the higher pressure flowing air gets sucked into.

With the rounded corners I would have expected that there would be some tendancy for the airflow to adhere to the contours, leading to a slightly smoother flow.