Impedance is not constant; never has been. Resistance is constant. True impedance is resistance and reactance, factoring in the electrical and mechanical forces acting upon and generated by the driver. It varies with frequency, sometimes greatly, and the mechanical and electrical resonances of a driver and cabinet. What the rating gives is a "range" that so long as the driver's tuning parameters are obeyed, will not exhibit potentially damaging resonant impedance spikes. Typically for bass drivers, as frequency decreases, impedance increases. This occurs because low frequencies require a much higher amount of current to sustain them and the physical excursion of the driver increases. This is the reasons lows are so hard to reproduce. In a sealed cabinet, the impedance to frequency ratio is an almost perfect 45 degree angle, but when you add things like ports and vents, things can get REALLY wacky.
