This image shows modeled baffle step and diffraction differences between the olive shape and the square edges. The edges are causing edge diffraction that is delivering the dip at 3K.
The purple line is the model of the square box.
Now this does not mean you have to make an olive shaped box to get the best sound. The olive shape does help with baffle edge diffraction, but my square box test is a worse case scenario. So as long as you dont make your baffle a perfect square and mount the driver right dead in the middle you should be able to reduce the 3k dip that i have shown above.