Those recessed edges have been addressed by the Aperion Engineering dept. and you are 100% correct they are in the measured plots for FR, as I just got off the phone w/ Aperion.
Oliver at Aperion states that the best measurements are with the Grills ON, not off, and that will work fine with me as I want the grills on.
BTW -- they recommend using a Microfiber cloth to clean the driver cones of any dust.
OK Facts --
Knowing the Speed of Sound = 1087 ft./sec. for medium=air, at 0 deg. C
So, for 20kHz.
(1087 ft/s) / (20,000 cyc/s) = [0.05435 ft. x 12 in./ft.] = 0.6522 inches [knowing cycles like radians are Dimensionless]
Now, even with a 0.25 inch recess on the front baffles edges, the wavelength at 20kHz. being much larger than the recess will not effect it.
Hopefully, this will technically explain that issue, but I am sure it should open up another Audiophiles perspective.
I WOULD LOVE TO HEAR DENNIS MURPHY'S EXPERIENCE IN THIS AREA OF EXPERTISE !!