You simply have multiple transition points from higher to lower velocity. All 90 degree bends, the mouth of each individual port and the mouth of the single port. They each shift the resonant impedance to give you a specific impedance curve. That will change with each type of vehicle you put it in and where/which direction you face it. Of course, that changes with whichever drivers you choose also. At this point, its a box. Any matching pair of subs you throw in it will have a different response curve than the next matching pair of subs will.