I ponder if the square peg in a true tension situation would enhance the relish to shear out easier. Due to the two sharp corners making nice starts for the relish to blow out. I picture a round hole confuses the stresses and can't find a good starting point. Or at least not two clear starting points, only one from the center of the round hole. A similar effect is seen in curves cut with an adze on the under sides of joist, spreading out the shear point, as opposed to a square reduction or even a 45.
