Pyramid shaped - different surfaces for sound waves of varying frequencies to reflect off and be possibly absorbed before transmitting through. All surfaces flat, so all waves incident at the same angle will be reflected in the same trajectory

Curvature result in them being reflected in a wide range of trajectories. For each frequency, there is a certain amount of thickness that allows for the most amount of sound to be absorbed/reflected. With more trajectories, the overall absorption of the sound will be increased. Also with more reflection, the sound wave will gradually lose energy as it travels through the air within the box due to being converted to heat (drag).

More trajectories also result in a greater spread of the sound across the sides of the box. Since the measurement was conducted on one specific side of the box, when it's dispersed, the dB registered would understandably be less. However, since it is for soundproofing in a household, most of each sound source would generally come one direction, and any sound dispersed on the other areas would have to take a greater distance to travel to the detector or other room (weaker due to the inverse square law).

Refraction that occurs on a semicircular wall converges the sound wave near the focus, which is very near to the wall itself, resulting in destructive interference and more absorption too.

For the holes, not only is there more curvature due to them being circular, sound waves trapped within the holes at just the right angle will infinitely bounce within it and eventually be absorbed or dissipated as heat. And there are a shit ton of holes so the odds aren't that low.