The alignment of a bass-reflex speaker cabinet sets a handful of significant frequencies to obtain a desirable overall response curve. The resonant frequency of the cone is shifted by the stiffness of the box. The box itself is another tuned system. And the interaction of these two tuned systems being so tightly coupled gives rise to two other significant frequencies, fl and fh. There's more about this in the post called "Behaviour of vented loudspeaker systems."